The regular expression is capturing the first string it encounters that does not contain a slash. That first string is "http:". Assuming you actually want to capture the last one, you can use the $ metacharacter to anchor the regular expression to the end of the string:

if($mystring =~ m!([^/]+)$!){

More information on metacharacters can be found in Regular Expressions in perlre or Simple word matching in perlretut.