Glad you're going to avoid that "feature" :-) (see e.g. Re: static-like persistence of my variable due to trailing conditional and How does my work with a trailing conditional for previous discussions)

I'd like to give you two reasonable workarounds. The first is simply using the global variable, with my added suggestion to enclose it in an anonymous block: 

{ # limit scope
    my @o;
    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 "Matches: @m";
        print "Offsets: @o";
        print " ";
    }
}

[download]
This will make sure that only foo() can see @o.

The second workaround is basically a rewrite of your code. It doesn't solve the general issue, but it avoids the use of the complicated (?{BLOCK}) feature for your particular case: 

sub foo {
    my $window = "a b X20 c X5 d e X17 X12";
    my( @o, @m );
    while( $window =~ m/(X\d+)/g ) {
        push @m, $1;
        push @o, $+[0];
    }
    
    print "Matches: @m";
    print "Offsets: @o";
    print " ";
}

[download]

In reply to Re^4: Multiple uses of (?{ code }) do not appear to be called by rhesa
in thread Multiple uses of (?{ code }) do not appear to be called by bsdz

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