*if* rexec is allowed, that is the most simple solution.

# perl -MNet::Rexec=rexec -le"my ($rc, @out) = rexec ("remote_host", "df -k", "username", "password");
But chances are that that connection is refused, as rexec is not commonly allowed. If it works, you've got the output of the command df -k in @out.

In script that might look like:

use strict;
use warnings;

use Net::Netrc;
use Net::Rexec qw( rexec );

foreach my $host (qw( host1 host2 host3 )) {
    my $machine;
    unless ($machine = Net::Netrc->lookup ($host)) {
        warn "Machine $host not found in .netrc\n";
        next;
        }
    my ($user, $password) = $machine->lpa;
    my ($rc, @out) = rexec ($host, "df -k", $user, $password);
    foreach my $df_line (@out) {
        # ...
        }
    }
[download]

Net::SNMP might suffer the same problem. Not all hosts have the protocol enabled (open in the firewall), and if they do, you would need to find the right setup parameters, which is not always easy. I don't think that if you say you are a real beginner, Net::SNMP is the correct module to start with.