my %session_params = (
    hostname      => $ARGV[0],
    port          => $ARGV[6],
    version       => $ARGV[5] ? "snmpv" . $ARGV[5] : '',
    timeout       => $ARGV[7],
    ... # and so on for all params
};

for ( keys( %session_params ) {
    delete( $session_params{$_} unless $session_params{$_};
}

my ( $session, $error ) = Net::SNMP->session( %session_params );

# Update:
# allowed for an empty string for an arg that was being manipulated: $ARGV[5]