Returns the length in characters of the value of EXPR. If EXPR is omitted, returns the length of $_. If EXPR is undefined, returns undef.
This function 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.
Like all Perl character operations, length normally deals in logical characters, not physical bytes. For how many bytes a string encoded as UTF-8 would take up, use length(Encode::encode_utf8(EXPR)) (you'll have to use Encode first). See Encode and perlunicode.