There are two syntaxes for calling map. If you are passing a block of code, there is no comma between the block and the list. If you are passing an expression, there is a comma. In this case, since it's just a string, I'd recommend the expression form.
Watch out if your expression is surrounded by parentheses, though, because map will think they enclose all of its arguments. You either need two sets of parens (one around your expression, and one around all of map's args), or you don't need any. In your case, you don't need any.
return map "$url/$_\n", @list;
Caution: Contents may have been coded under pressure.
