To clarify what hexcoder said...

Based on the documentation, $Net::SNMP::Interfaces::error is only set if you haven't used RaiseError => 1. You should be able to do one of the following:

#!/usr/bin/perl
use strict;
use warnings;
use Net::SNMP::Interfaces;
use Data::Dumper qw(Dumper);

my $interfaces = Net::SNMP::Interfaces->new(
                         Hostname   => '127.0.0.1',
                         Community  => 'public',
                         Port       => '1161',
                         RaiseError => 1, # Errors are fatal
    );

my @ifnames = $interfaces->all_interfaces();

print Dumper (\@ifnames);
[download]

#!/usr/bin/perl
use strict;
use warnings;
use Net::SNMP::Interfaces;
use Data::Dumper qw(Dumper);

my $interfaces = Net::SNMP::Interfaces->new(
                         Hostname   => '127.0.0.1',
                         Community  => 'public',
                         Port       => '1161',
    ) or die $Net::SNMP::Interfaces::error;

my @ifnames = $interfaces->all_interfaces();

print Dumper (\@ifnames);
[download]