"debconf-communicate dumped core". What sends QUIT ("signal 3")?

(I have installed systemd-coredump) What would send a QUIT signal to debconf-communicate? Since the backtrace does not include raise() / kill(), I guess I can assume this fatal signal was sent to debconf-communicate by some other running program? man 7 signal says that signal QUIT was originally intended for "Quit from keyboard". The distinction from signal INT ("Interrupt from keyboard") is that QUIT deliberately dumps core. Presumably this was a debugging technique. SIGINT is sent by the key combination Ctrl+C. SIGQUIT is sent by Ctrl+\\. However I seem to be getting these core dumps without anyone pressing Ctrl+\\ on an apt command running in a terminal. It seems to be happening (sometimes?) when doing package upgrades with PackageKit. This is a Debian 9 desktop system (GNOME). I don't know what happens at the times when this crash occurs, other than to say the users have not reported any problem :-). The main user is generally good at applying updates, and understands their importance. Due to bugs in Debian which failed to deliver update notifications in the past, the system currently has installed pk-update-icon (with .desktop file patched to auto-start in GNOME) and gnome-packagekit / gpk-update-viewer, in addition to the usual gnome-software.

$ dpkg-query -l packagekit gnome-software gnome-packagekit pk-update-icon debconf dpkg apt
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                 Version         Architecture    Description
+++-====================-===============-===============-==============================================
ii  apt                  1.4.9           amd64           commandline package manager
ii  debconf              1.5.61          all             Debian configuration management system
ii  dpkg                 1.18.25         amd64           Debian package management system
ii  gnome-packagekit     3.22.1-2        amd64           Graphical distribution neutral package manager
ii  gnome-software       3.22.5-1        amd64           Software Center for GNOME
ii  packagekit           1.1.5-2+deb9u1  amd64           Provides a package management service
ii  pk-update-icon       2.0.0-2         amd64           Displays an update-notification tray icon

$ sudo coredumpctl --no-pager
TIME                            PID   UID   GID SIG COREFILE EXE
Thu 2018-07-12 10:13:09 BST    5059  1005  1009   3 missing  /usr/bin/perl
Sat 2018-07-14 11:16:38 BST    2346  1005  1009   3 missing  /usr/bin/perl
...
Wed 2019-05-22 14:35:32 BST    2535  1005  1009   3 missing  /usr/bin/perl
Thu 2019-05-23 11:39:52 BST    2311  1005  1009   3 missing  /usr/bin/perl
Sat 2019-05-25 10:16:47 BST    2445  1005  1009   3 missing  /usr/bin/perl
Thu 2019-05-30 08:16:50 BST    2497  1005  1009   3 missing  /usr/bin/perl
Fri 2019-05-31 11:10:27 BST    2404  1005  1009   3 missing  /usr/bin/perl
Thu 2019-06-06 15:47:50 BST    4282  1005  1009   3 missing  /usr/bin/perl
Fri 2019-06-07 10:16:49 BST    2640  1005  1009   3 missing  /usr/bin/perl

$ sudo coredumpctl info 2640
           PID: 2640 (debconf-communi)
           UID: 1005 (YYY)
           GID: 1009 (YYY)
        Signal: 3 (QUIT)
     Timestamp: Fri 2019-06-07 10:16:49 BST (4 days ago)
  Command Line: /usr/bin/perl -w /usr/bin/debconf-communicate
    Executable: /usr/bin/perl
 Control Group: /user.slice/user-1005.slice/session-2.scope
          Unit: session-2.scope
         Slice: user-1005.slice
       Session: 2
     Owner UID: 1005 (YYY)
       Boot ID: 37f2efd8086a42b5a39144b5411b1523
    Machine ID: 35fb9280fa2847946f17a060542fd617
      Hostname: drystone
       Storage: /var/lib/systemd/coredump/core.debconf-communi.1005.37f2efd8086a42b5a39144b5411b1523.2640.1559899009000000000000.lz4 (inaccessible)
       Message: Process 2640 (debconf-communi) of user 1005 dumped core.
                
                Stack trace of thread 2640:
                #0  0x00007fa8db86a210 read (libpthread.so.0)
                #1  0x00005598354afa7d PerlIOUnix_read (perl)
                #2  0x00005598354b2fcf PerlIOBuf_fill (perl)
                #3  0x00005598354b1658 Perl_PerlIO_fill (perl)
                #4  0x00005598354b2e80 PerlIOBase_read (perl)
                #5  0x00005598354b4e08 PerlIO_getc (perl)
                #6  0x000055983544b2a5 Perl_sv_gets (perl)
                #7  0x000055983542d376 Perl_do_readline (perl)
                #8  0x0000559835428aa6 Perl_runops_standard (perl)
                #9  0x00005598353ae8a9 perl_run (perl)
                #10 0x0000559835387a5d main (perl)
                #11 0x00007fa8db4db2e1 __libc_start_main (libc.so.6)
                #12 0x0000559835387a9a _start (perl)

$ sudo journalctl --since="2019-06-07"
...
Jun 07 10:16:05 drystone PackageKit: get-updates transaction /21666_bacedcbd from uid 1005 finished with success after 657ms
Jun 07 10:16:09 drystone PackageKit: new update-packages transaction /21669_aeeccbac scheduled from uid 1005
Jun 07 10:16:10 drystone PackageKit: get-update-detail transaction /21667_cebbebab from uid 1005 finished with success after 5406ms
Jun 07 10:16:11 drystone PackageKit: get-details transaction /21668_dabdcebe from uid 1005 finished with success after 395ms
Jun 07 10:16:11 drystone PackageKit: in /21669_aeeccbac for update-packages package evolution;3.22.6-1+deb9u2;amd64;debian-stable-main was updating for uid 1005
Jun 07 10:16:11 drystone PackageKit: in /21669_aeeccbac for update-packages package evolution-common;3.22.6-1+deb9u2;all;debian-stable-main was updating for uid 1005
Jun 07 10:16:11 drystone PackageKit: in /21669_aeeccbac for update-packages package evolution-plugins;3.22.6-1+deb9u2;amd64;debian-stable-main was updating for uid 1005
Jun 07 10:16:11 drystone PackageKit: in /21669_aeeccbac for update-packages package libevolution;3.22.6-1+deb9u2;amd64;debian-stable-main was updating for uid 1005
Jun 07 10:16:11 drystone PackageKit: in /21669_aeeccbac for update-packages package qemu-kvm;1:2.8+dfsg-6+deb9u7;amd64;debian-stable-main was updating for uid 1005
Jun 07 10:16:11 drystone PackageKit: in /21669_aeeccbac for update-packages package qemu-system-common;1:2.8+dfsg-6+deb9u7;amd64;debian-stable-main was updating for uid 1005
Jun 07 10:16:11 drystone PackageKit: in /21669_aeeccbac for update-packages package qemu-system-x86;1:2.8+dfsg-6+deb9u7;amd64;debian-stable-main was updating for uid 1005
Jun 07 10:16:11 drystone PackageKit: in /21669_aeeccbac for update-packages package qemu-utils;1:2.8+dfsg-6+deb9u7;amd64;debian-stable-main was updating for uid 1005
Jun 07 10:16:11 drystone PackageKit: update-packages transaction /21669_aeeccbac from uid 1005 finished with success after 585ms
Jun 07 10:16:11 drystone PackageKit: uid 1005 is trying to obtain org.freedesktop.packagekit.system-update auth (only_trusted:1)
Jun 07 10:16:11 drystone gnome-shell: JS LOG: polkitAuthenticationAgent: Received 2 identities that can be used for authentication. Only considering one.
Jun 07 10:16:16 drystone polkitd(authority=local): Operator of unix-session:2 successfully authenticated as unix-user:YYY to gain TEMPORARY authorization for action org.freedesktop.packagekit.system-update
Jun 07 10:16:16 drystone PackageKit: new update-packages transaction /21670_adbeabac scheduled from uid 1005
Jun 07 10:16:16 drystone PackageKit: uid 1005 obtained auth for org.freedesktop.packagekit.system-update
Jun 07 10:16:38 drystone packagekitd: PARENT process running...
Jun 07 10:16:41 drystone gnome-software-service.desktop: 09:16:41:0610 As  failed to rescan: No valid root node specified
Jun 07 10:16:41 drystone gnome-software-service.desktop: 09:16:41:0617 As  failed to rescan: Failed to parse /usr/share/applications/evolution.desktop file: cannot process file of type application/x-deskto
Jun 07 10:16:41 drystone gnome-software-service.desktop: 09:16:41:0642 As  failed to rescan: No valid root node specified
Jun 07 10:16:41 drystone gnome-software-service.desktop: 09:16:41:0646 As  failed to rescan: Failed to parse /usr/share/applications/evolution.desktop.dpkg-tmp file: cannot process file of type text/plain
Jun 07 10:16:42 drystone gnome-software-service.desktop: 09:16:42:0406 As  failed to rescan: No valid root node specified
Jun 07 10:16:42 drystone gnome-software-service.desktop: 09:16:42:0410 As  failed to rescan: No valid root node specified
Jun 07 10:16:42 drystone gnome-software-service.desktop: 09:16:42:0410 As  failed to rescan: No valid root node specified
Jun 07 10:16:42 drystone gnome-software-service.desktop: 09:16:42:0414 As  failed to rescan: No valid root node specified
Jun 07 10:16:42 drystone gnome-software-service.desktop: 09:16:42:0414 As  failed to rescan: No valid root node specified
Jun 07 10:16:44 drystone gnome-software-service.desktop: 09:16:44:0075 As  failed to rescan: No valid root node specified
Jun 07 10:16:44 drystone gnome-software-service.desktop: 09:16:44:0078 As  failed to rescan: No valid root node specified
Jun 07 10:16:44 drystone gnome-software-service.desktop: 09:16:44:0082 As  failed to rescan: No valid root node specified
Jun 07 10:16:49 drystone packagekitd: Parent finished...
Jun 07 10:16:49 drystone PackageKit: in /21670_adbeabac for update-packages package qemu-system-common;1:2.8+dfsg-6+deb9u7;amd64;debian-stable-main was installing for uid 1005
Jun 07 10:16:49 drystone PackageKit: in /21670_adbeabac for update-packages package evolution-plugins;3.22.6-1+deb9u2;amd64;debian-stable-main was installing for uid 1005
Jun 07 10:16:49 drystone PackageKit: in /21670_adbeabac for update-packages package evolution;3.22.6-1+deb9u2;amd64;debian-stable-main was installing for uid 1005
Jun 07 10:16:49 drystone PackageKit: in /21670_adbeabac for update-packages package libevolution;3.22.6-1+deb9u2;amd64;debian-stable-main was installing for uid 1005
Jun 07 10:16:49 drystone PackageKit: in /21670_adbeabac for update-packages package evolution-common;3.22.6-1+deb9u2;all;debian-stable-main was installing for uid 1005
Jun 07 10:16:49 drystone PackageKit: in /21670_adbeabac for update-packages package qemu-kvm;1:2.8+dfsg-6+deb9u7;amd64;debian-stable-main was installing for uid 1005
Jun 07 10:16:49 drystone PackageKit: in /21670_adbeabac for update-packages package qemu-system-x86;1:2.8+dfsg-6+deb9u7;amd64;debian-stable-main was installing for uid 1005
Jun 07 10:16:49 drystone PackageKit: in /21670_adbeabac for update-packages package qemu-utils;1:2.8+dfsg-6+deb9u7;amd64;debian-stable-main was installing for uid 1005
Jun 07 10:16:49 drystone PackageKit: in /21670_adbeabac for update-packages package evolution-common;3.22.6-1+deb9u2;all;debian-stable-main was installing for uid 1005
Jun 07 10:16:49 drystone PackageKit: in /21670_adbeabac for update-packages package libglib2.0-0;2.50.3-2;amd64;installed:debian-stable-main was installing for uid 1005
Jun 07 10:16:49 drystone PackageKit: in /21670_adbeabac for update-packages package qemu-utils;1:2.8+dfsg-6+deb9u7;amd64;debian-stable-main was installing for uid 1005
Jun 07 10:16:49 drystone PackageKit: in /21670_adbeabac for update-packages package libevolution;3.22.6-1+deb9u2;amd64;debian-stable-main was installing for uid 1005
Jun 07 10:16:49 drystone PackageKit: in /21670_adbeabac for update-packages package qemu-system-common;1:2.8+dfsg-6+deb9u7;amd64;debian-stable-main was installing for uid 1005
Jun 07 10:16:49 drystone PackageKit: in /21670_adbeabac for update-packages package qemu-system-x86;1:2.8+dfsg-6+deb9u7;amd64;debian-stable-main was installing for uid 1005
Jun 07 10:16:49 drystone PackageKit: in /21670_adbeabac for update-packages package evolution;3.22.6-1+deb9u2;amd64;debian-stable-main was installing for uid 1005
Jun 07 10:16:49 drystone PackageKit: in /21670_adbeabac for update-packages package evolution-plugins;3.22.6-1+deb9u2;amd64;debian-stable-main was installing for uid 1005
Jun 07 10:16:49 drystone PackageKit: in /21670_adbeabac for update-packages package qemu-kvm;1:2.8+dfsg-6+deb9u7;amd64;debian-stable-main was installing for uid 1005
Jun 07 10:16:49 drystone PackageKit: update-packages transaction /21670_adbeabac from uid 1005 finished with success after 33545ms
Jun 07 10:16:49 drystone gpk-update-view: Source ID 1825 was not found when attempting to remove it
Jun 07 10:16:49 drystone systemd: Created slice system-systemd\x2dcoredump.slice.
Jun 07 10:16:49 drystone systemd: Started Process Core Dump (PID 3620/UID 0).
Jun 07 10:16:49 drystone systemd-coredump: Process 2640 (debconf-communi) of user 1005 dumped core.
                                                 
                                                 Stack trace of thread 2640:
                                                 #0  0x00007fa8db86a210 read (libpthread.so.0)
                                                 #1  0x00005598354afa7d PerlIOUnix_read (perl)
                                                 #2  0x00005598354b2fcf PerlIOBuf_fill (perl)
                                                 #3  0x00005598354b1658 Perl_PerlIO_fill (perl)
                                                 #4  0x00005598354b2e80 PerlIOBase_read (perl)
                                                 #5  0x00005598354b4e08 PerlIO_getc (perl)
                                                 #6  0x000055983544b2a5 Perl_sv_gets (perl)
                                                 #7  0x000055983542d376 Perl_do_readline (perl)
                                                 #8  0x0000559835428aa6 Perl_runops_standard (perl)
                                                 #9  0x00005598353ae8a9 perl_run (perl)
                                                 #10 0x0000559835387a5d main (perl)
                                                 #11 0x00007fa8db4db2e1 __libc_start_main (libc.so.6)
                                                 #12 0x0000559835387a9a _start (perl)
Jun 07 10:16:50 drystone PackageKit: get-updates transaction /21671_baceadea from uid 1005 finished with success after 597ms
Jun 07 10:16:50 drystone gpk-update-view: gtk_widget_hide: assertion 'GTK_IS_WIDGET (widget)' failed
Jun 07 10:16:52 drystone PackageKit: get-updates transaction /21672_cdcdcbca from uid 1005 finished with success after 884ms
Jun 07 10:16:52 drystone gpk-update-view: gtk_widget_hide: assertion 'GTK_IS_WIDGET (widget)' failed
Jun 07 10:16:55 drystone gpk-update-view: g_application_release: assertion 'application->priv->use_count > 0' failed
Jun 07 10:16:57 drystone PackageKit: get-updates transaction /21673_bdabdecd from uid 1005 finished with success after 638ms
Jun 07 10:17:01 drystone CRON: pam_env(cron:session): Unable to open env file: /etc/environment: No such file or directory
Jun 07 10:17:01 drystone CRON: pam_unix(cron:session): session opened for user root by (uid=0)
Jun 07 10:17:01 drystone CRON: (root) CMD (   cd / && run-parts --report /etc/cron.hourly)
Jun 07 10:17:01 drystone CRON: pam_unix(cron:session): session closed for user root
Jun 07 10:24:14 drystone systemd: Starting Cleanup of Temporary Directories...
Jun 07 10:24:14 drystone systemd: Started Cleanup of Temporary Directories.