you could use a simple regex of course:

$_="http://bla.com";
s#([^/])$#$1/#; 
$_ .= "foo.html";
print;
# prints http://bla.com/foo.html


$_="http://bla.com/";
s#([^/])$#$1/#; 
$_ .= "foo.html";
print;
# prints http://bla.com/foo.html
[download]

always a million ways to do things ;-)