I'll gladly accept your advice about it being bad style, but I don't understand why you say my code would break. Given that the following works, I must be overlooking something:

#!/usr/bin/perl -l

use strict;

bar();
bar();

{    # limit scope
    my @o;
    sub bar
    {
        my $window = "a b X20 c X5 d e X17 X12";
        @o = ();
        my @m = ( $window =~ m/(X\d+(?{push @o, pos()}))/g );
        print "Matches: @m";
        print "Offsets: @o";
        print " ";
    }
}

print "One last time";
bar();

__END__
Matches: X20 X5 X17 X12
Offsets: 7 12 20 24
 
Matches: X20 X5 X17 X12
Offsets: 7 12 20 24
 
One last time
Matches: X20 X5 X17 X12
Offsets: 7 12 20 24
[download]

Even if I put the bar() call inside the bare block, right after the sub definition, it still works. I'd appreciate it if you could enlighten me.