Esteemed monks

A question for your perusal. The following code: - 

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");
}

[download]
Gives the following output: - 
Matches: X20 X5 X17 X12
Offsets: 7 12 20 24

Matches: X20 X5 X17 X12
Offsets:

Matches: X20 X5 X17 X12
Offsets:

[download]
Is there any reason for the missing offsets on the 2nd and 3rd runs? The output is the same for Cygwin and AS Perl.

In reply to Multiple uses of (?{ code }) do not appear to be called by bsdz

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