Something

#    RewriteRule     ^(pl(/.*)?)$           mythweb.pl?PATH_INFO=/$1    [L,QSA]
#    RewriteRule     ^(.+)$                 mythweb.php?PATH_INFO=/$1   [L,QSA]

# Catch anything else that comes through and send it to mythweb.php with no parameters.
    RewriteRule     ^(.*)$                  mythweb.php                 [QSA,L]