You could try using the experimental (and very-easy-to-break, in my experience) pretend-fork of Win32 Perl.

But I'd just do something like:

if(  @ARGV  &&  $ARGV[0] eq "do_long_sub"  ) {
    &long_sub();
    exit( 0 );
}
# ...
$main->button(
    -text => "Start Long-Sub",
    -command => sub { system(1,$^X,$0,"do_long_sub") }
)->pack;
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        - tye (but my friends call me "Tye")