I would write is as follows:

#!/usr/bin/perl -w
     
use strict;
use Net::SNMP;

Usage () unless @ARGV == 2; 

my ($session, $error) = Net::SNMP -> session (
    -hostname  => $ARGV [0],
    -community => $ARGV [1],
);

$session or die "Session error: $error";
                
my $tableid = "1.3.6.1.4.1.1991.1.1.4.6.1.1";
my $table   =  $session -> get_table ($tableid)
                or die "request error: ", $session -> error;
foreach my $oid (keys %$table) {
    if ($oid =~ /^\Q$tableid\E\.(\d+)/) {
        print "Real server ",                  $table -> {$oid},
              ", port ",                       $table -> {"$tableid.5.
+$1"},
              ", is bound to virtual server ", $table -> {"$tableid.2.
+$1"},
              ", on port ",                    $table -> {"$tableid.3.
+$1"},
              "\n";
    }
}

my $sysid      = "1.3.6.1.2.1.1.4.0";
my $syscontact = $session -> get_request ($sysid)
                  or die "Request error: ", $session -> error;
print "For help or questions contact: ", $syscontact -> {$sysid}, "\n"
+;

$session -> close;
[download]

Some differences: no global variables (with use vars). No magical constants that are repeated. Taken care of the special meaning of a dot inside a regex. Anchored the regex. Used parenthesis to capture relevant field. No needless concatenation in print or die. Success is checked right away and not in a separate statement. Just one check to see if there are enough arguments.

-- Abigail