It's much easier, safer and faster to use package variables when dealing with regexps.

sub foo {
    my $window = "a b X20 c X5 d e X17 X12";
    local our @o;
    my @m = ($window =~ m/(X\d+(?{ push @o, pos }))/g);
    print join(" ", "Matches:", @m, "\n");
    print join(" ", "Offsets:", @o, "\n\n");
}
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Update: Oops, this has already been said elsewhere in the thread. I didn't enter this thread at the root node.