To debug your script, you can start by adding verbose and debugging:

#!/usr/bin/perl

use strict;
use warnings;
use Devel::SimpleTrace;
use SNMP;

$SNMP::verbose   = 1;
$SNMP::debugging = 1;

$ENV{'MIBS'} = 'ALL';
&SNMP::initMib();

my $session = new SNMP::Session(DestHost => $ARGV[0], Community =>$ARG
+V[1]);
my $numInts = $session->get('1.3.6.1.2.1.2.1.0');
print "Number of Interfaces: $numInts\n";
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That'll tell you why you're not getting back any values.

Here's a different script with errors:

#!/usr/bin/perl

use strict;
use warnings;
use SNMP;

my $hostname  = 'localhost';
my $port      = 161,
my $community = 'public';

$SNMP::verbose     = 1;
$SNMP::debugging   = 1;
$SNMP::dump_packet = 1;

my $session = new SNMP::Session(
              'DestHost'     => $hostname, 
              'Community'    => $community,
              'RemotePort'   => $port,
              'UseSprintValue' => 1 );    
die "session creation error: Can't fire-up!\n" 
unless (defined $session); 

my $vars = new SNMP::VarList(['ipNetToMediaNetAddress'], 
           ['ipNetToMediaPhysAddress']); 

my  ($ip,$mac) = $session->getnext($vars); 
die $session->{ErrorStr} if ($session->{ErrorStr}); 

while (!$session->{ErrorStr} and $$vars[0]->tag eq 
"ipNetToMediaNetAddress") { 
    print "$ip -> $mac\n"; 
};
($ip,$mac) = $session->getnext($vars);
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SNMP is just one of those modules that have a steep learning curve. It's also extremely difficult to get it to work properly, at least for me.

As for interfaces, I don't think that SNMP will get that for you. Instead, try SNMP::Info. Here's an example using SNMP::Info::Layer3::NetSNMP:

#!/usr/bin/perl

use strict;
use warnings;
use SNMP::Info;

$SNMP::verbose   = 1;
$SNMP::debugging = 1;

my $netsnmp = new SNMP::Info(
              AutoSpecify => 1,
              Debug       => 1,
              DestHost    => 'myrouter',
              Community   => 'public',
              Version     => 3
              )
    or die "Can't connect to DestHost.\n";
    
my $class = $netsnmp->class();
print "SNMP::Info determined this device to fall
       under subclass: $class\n";
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