I always test the returned varbind to ensure the OID hasn't incremented past the branch that I want. (getnext always gets the next OID regardless of what the initial OID was). (OID = object identifier, it's the .1.3.6.1... thing)

  my $baseoid = ".1.3.6.1.2.1.2.2"; # ifTable
  my $thisoid = $baseoid;
  while ( 1 ) {
    $val = $sess->getnext($thisoid);
    last if ( $thisoid !~ m/^$baseoid\./ );
    last if ( $sess->{ErrorNum} );

    print( "@{$thisoid}\n" );
  }
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Update: I always use Net::SNMP, too. Why? Because it's pure Perl, unlike SNMP which relies on the UCD tools. The latest Net::SNMP, however, does some tricky SNMPv3 stuff that requires a recent version of Perl, making it not that useful for running on old installations of 5.005.. in which case getting an older copy of Net::SNMP isn't a bad idea.