Since your s/// works at the end of a string, you could just append to it, with some application of the ? : operator to check if there is a '/' at the end.

$thebase .= 
    (substr($thebase, -1) eq '/') ? 
    'default' : 
    '/default';
[download]

Which is functionally equivilent to:

if(substr($thebase, -1) eq '/') { 
    $thebase = $thebase . 'default';
}
else {
    $thebase = $thebase . '/default';
}
[download]

----
I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
-- Schemer

Note: All code is untested, unless otherwise stated