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]

##</code><code>##

#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

my $foo = 1;
my $bar = 'baz';
my $baz = '';

my %foohash = (
    $foo ? ( foo => $foo ) : (),
    bar => $bar,
    $baz ? ( baz => $baz ) : (),
    $ARGV[0] ? ( version => 'snmpv' . $ARGV[0] ) : ()
);

print Dumper(\%foohash);

exit;

__END__

Output:
$VAR1 = {
          'bar' => 'baz',
          'foo' => 1
        };