# assume that "$octets" is the string that has been recieved 
# from a form, and is purported to be utf8 text:
...
use Encode;
...

my $utf8str;
eval "\$utf8str = encode( 'utf8', \$octets, Encode::FB_CROAK )";
if ( $@ ) {
   # $octets was not really a valid utf8 string
}
...