#when you create the FORM hash
my @genbank_info= $query->param('genbank_info');
my %genbank_info= map { $_ => 1 } @genbank_info; # create a hash selected_value => 1
$FORM{genbank_info}= \%genbank_info;
...
# later...
if ($FORM{genbank_info}->{gi_number}) {
    $gi_number = "1234";
}