use warnings;
use strict;
use Tk;

my $string = "foobarbaz";

my $mw = MainWindow->new;

my $regexp_entry = $mw -> Entry() -> pack;
my $go_button = $mw -> Button(
    -takefocus => 1,
    -text => "Match using what you typed!",
    -command => sub{
        my $exp = $regexp_entry -> get();
        print STDOUT "I'm looking for a match for $exp...\n";
        $string =~ m/$exp/ ? print STDOUT "match!\n" : print STDOUT "N
+o match!\n";
    }
)-> pack;
my $used_stored_exp = $mw -> Button(
    -text => "Match with Stored Regexp!",
    -command => sub{
        my $exp = qr/foo/;
        print STDOUT "I'm looking for a match for $exp...\n";
        $string =~ m/$exp/ ? print STDOUT "match!\n" : print STDOUT "N
+o match!\n";
    }
)-> pack;
$regexp_entry -> focus;

MainLoop;
[download]

To see the failure, run the script and do the following:
(1)press the "Match using stored Regexp" button, and watch your STDOUT line.
(2)type

qr/foo/
[download]

into the entry widget, press the "Match using what you typed!" button, and watch STDOUT.

Surprisingly (perhaps only to a neophyte like me) it "works" if in step (2) above, you instead type

(?-xism:foo)
[download]

into the entry widget.

What's going on here? My first guess is that there's something happening with the entry widget's "get" method

Thanks!