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    our $pre;
    my $prefix;
...
        (\d+)}x;
    my $suffix = $3;
    say $match ? $prefix . $suffix : '';

    my $match = $str =~ m{^.{$pos}
        (?{local $pre = ''})
...
            (?{$prefix = $pre})
        )
        (\d*)}x;

    my ($start, $end);
    my $res = $str =~ m{
...
        )
        (\d*)(?{$end = pos})
    }x ? substr($str, $start, $end - $start) : undef ;