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 number of characters, not the number of bytes. To get the length in bytes, use do { use bytes; length(EXPR) }, see bytes.