#!/usr/bin/perl

use strict;
use warnings;

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

foo;
foo;
foo;

__END__