The 2.6.1 version of that section is :

        $message =~
 s/([^\w\"\=\[\]]|[\n\b]|\&quot\;|\[quote.*?\]|\[edit\]|\[highlight\]|\[\*\]|\[td\]|\A)\\*(\w+?\:\/\/(?:[\w\~\;\:\,\$\-\+\!\*\?\/\=\&\@\#\%\(\)\[\](?:\<\S+?\>\S+?\<\/\S+?\>)]+?)\.(?:[\w\~\.\;\:\,\$\-\+\!\*\?\/\=\&\@\#\%\(\)\[\]\x80-\xFF]{1,})+?)/format_url($1,$2)/eisgm;
         $message =~
 s/([^\"\=\[\]\/\:\.\-(\:\/\/\w+)]|[\n\b]|\&quot\;|\[quote.*?\]|\[edit\]|\[highlight\]|\[\*\]|\[td\]|\A|\()\\*(www\.[^\.](?:[\w\~\;\:\,\$\-\+\!\*\?\/\=\&\@\#\%\(\)\[\](?:\<\S+?\>\S+?\<\/\S+?\>)]+?)\.(?:[\w\~\.\;\:\,\$\-\+\!\*\?\/\=\&\@\#\%\(\)\[\]\x80-\xFF]{1,})+?)/format_url($1,$2)/eisgm; 

Without the 'my $reg1', 'my $reg2', 'my $reg3', and 'my $reg4' lines.

There are 5 files using the alnum posix:
Decoder, Display, Load, LogInOut, and YaBBC

In Display, Load, and LogInOut the posix has the last colon missing.

In Display the line containing the posix should be

$urlname =~ s/([^[:alnum:]])/sprintf('%%%02X', ord($1))/egxsm; 

In Load it should be

            s/%([[:alnum:]][[:alnum:]])/pack('C', hex($1))/egxsm; 

and in LogInOut it should be

    if ( $id =~ /[^[:alnum:]]/xsm ) { 

YaBBC and Decoder has the correct posix already. (Did you just use a global find and replace?  If so, you may have inadvertently broken YaBBC. )

I'm in the middle of a major database reconfiguration so I don't dare upgrade Perl to the newest version until that's done. (And it may be a while - 4000+ rows to be hand-edited.)

  It should be

[^[:alnum:]] 

The right colon is also missing in the

[:alnum 

in Load.pm and Display.pm

Since the problem doesn't throw an error here, I'm guessing that newer versions of Perl don't like it when the second colon is missing from the posix class while older versions son't care.