G'day Rob,

I don't know if any of the following will help, but here's some Perl, Tk and gcc version information for comparison. I've provided info for Perl 5.30 (which the OP had problems with) and Perl 5.36 (which you've had problems with) — note that I have different subversions.

I install Tk with all of my Perl versions. I usually get an error or two from `make test`, but nothing catastrophic; although, I seem to recall that I couldn't build Tk years ago (maybe Perl 5.26). I install with the cpan utility (force install Tk when tests fail). I run several of the widget demos, as additional tests, when `make test` fails.

I'm running Cygwin version 3.4.7 on Win10. I update both of these weekly; last done about seven hours ago.

Currently cmd.exe starts by showing:

Microsoft Windows [Version 10.0.19045.3155]
(c) Microsoft Corporation. All rights reserved.
[download]

Cygwin's latest version of gcc:

$ gcc --version
gcc (GCC) 11.4.0
[download]

I get Perl module version and location info with perlmodver:

Perl v5.30

$ perl -v | head -2 | tail -1
This is perl 5, version 30, subversion 0 (v5.30.0) built for cygwin-th
+read-multi

$ perl -V
Summary of my perl5 (revision 5 version 30 subversion 0) configuration
+:

  Platform:
    osname=cygwin
    osvers=3.0.7(0.33853)
    archname=cygwin-thread-multi
    uname='cygwin_nt-10.0 titan 3.0.7(0.33853) 2019-04-30 18:08 x86_64
+ cygwin '
...
    gccversion='7.4.0'
...

$ perlmodver Tk
Tk 804.034
  in /home/ken/perl5/perlbrew/perls/perl-5.30.0/lib/site_perl/5.30.0/c
+ygwin-thread-multi/Tk.pm
----------------------------------------------------------------------
+--
[download]

Full perl -V output:

Perl v5.36

$ perl -v | head -2 | tail -1
This is perl 5, version 36, subversion 0 (v5.36.0) built for cygwin-th
+read-multi

$ perl -V
Summary of my perl5 (revision 5 version 36 subversion 0) configuration
+:

  Platform:
    osname=cygwin
    osvers=3.2.0(0.34053)
    archname=cygwin-thread-multi
    uname='cygwin_nt-10.0 titan 3.2.0(0.34053) 2021-03-29 08:42 x86_64
+ cygwin '
...
    gccversion='10.2.0'
...

$ perlmodver Tk
Tk 804.036
  in /home/ken/perl5/perlbrew/perls/perl-5.36.0/lib/site_perl/5.36.0/c
+ygwin-thread-multi/Tk.pm
----------------------------------------------------------------------
+--
[download]

Full perl -V output: