while (<IN>) {  # first list
    next unless ( m:(.+?)#(\d+)$: );
    $pathname_versions{$1} = $2;
}


##</code><code>##

while (<IN>) {  # second list
    next unless ( m:(.+?)#(\d+)$: );
    ...
    if ( $pathname_versions{$1} < $2 ) { ...