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