{ # limit scope
    my @o;
    sub foo {
        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 " ";
    }
}