look at perlfunc:substr (particularly the negative examples)

also, look at perlre(to see what $ means in a regex), and perlop (for the m/// operator)

which should lead you to something like:

if(length $foo > 1 and substr($foo,-1,1) ne '/')
{ $foo .= '/'; }
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or

$foo .= '/' unless($foo =~ m/\/$/);
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btw - if you aren't used to the unless syntaxt above, see perlsyn