use Encode;
...
my ( $stringFromDB, $uft8string );
#
# do whatever it is that queries the database and
# assigns a string to $stringFromDB...
#
eval "\$utf8string = decode( 'utf8', \$stringFromDB, Encode::FB_CROAK )";

if ( $@ ) {
    warn "DB value $stringFromDB is Malformed UTF8\n";
}
...