sub foo {
    my $window = "a b X20 c X5 d e X17 X12";
    my @o = ();
    my @m = ($window =~ m/(X\d+(?{print ">pos:",pos();push @o, pos()}))/g);
    print "Matches: @m";
    print "Offsets: @o";
    print " ";
}
__END__
>pos:7
>pos:12
>pos:20
>pos:24
Matches: X20 X5 X17 X12
Offsets: 7 12 20 24
 
>pos:7
>pos:12
>pos:20
>pos:24
Matches: X20 X5 X17 X12
Offsets: 
 
>pos:7
>pos:12
>pos:20
>pos:24
Matches: X20 X5 X17 X12
Offsets: