I must admit I am not too hot on closures either but another interesting observation is that making @o global appears to cure the problem. 
my @o;
foo();
foo();
foo();

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

[download]
Could that be explained by closures too? I am still examining the re 'debug' output.

In reply to Re^2: Multiple uses of (?{ code }) do not appear to be called by bsdz
in thread Multiple uses of (?{ code }) do not appear to be called by bsdz

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