sub foo {
    my $window = "a b X20 c X5 d e X17 X12";
    my @o = () if pos; # conditional declaration makes @o "static"
    @o = (); # reset it always
    my @m = ($window =~ m/(X\d+(?{push @o, pos()}))/g);
    print "Matches: @m";
    print "Offsets: @o";
    print " ";
}

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