sub trim {
   for (my $s = $_[0]) {  # passed param assigned, not used
      s/^\s+//;           # substitutions on $_
      ...
[download]

Actually, it works... it's only written in a somewhat weird fashion :)

The passed parameter is assigned to $s, which is then aliased to $_ by the for loop... The seemingly superfluous $s is needed, because otherwise you'd get the error "Modification of a read-only value attempted".  But I agree there are more readable versions...