$utf16= pack "S*", unpack( "C*", $ascii ), 0;
chop( $ascii= pack "C*", unpack "S*", $utf16 );
Where "UTF16" refers to a 16-bit flavor of Unicode that I think is what Microsoft calls UNICODE (and is not very much like what most non-Microsoft references refer to as Unicode which I think is more precisely called UTF8).
Note how the UTF16 string explicitly includes the trailing L'\0' while the ASCII string does not explicitly include the trailing '\0'. This is because Perl strings implicitly include a trailing '\0' tacked on the end but this is not enough to terminate a 16-bit-char string.