foo();
foo();
foo();

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

##</code><code>##

Matches: X20 X5 X17 X12
Offsets: 7 12 20 24

Matches: X20 X5 X17 X12
Offsets:

Matches: X20 X5 X17 X12
Offsets: