perldoc -f length

   length EXPR
   length  Returns the length in characters of the value of EXPR.  If EXPR is
           omitted,returns length of $_.  Note that this cannot be used on an
           entire array or hash to find out how many elements these have.  For
           that,use "scalar @array" and "scalar keys %hash" respectively.

           Note the characters: if the EXPR is in Unicode,you will get the num-
           ber of characters,not the number of bytes.  To get the length in
           bytes,use "do { use bytes; length(EXPR) }",see bytes.