Oh, man. That's tight.

Here's a beginner's version:

use strict;
use warnings;
use 5.010;

my %HoH = (
    '192.168.0.1' => {
        user   => "fred",
        pass   => "barney",
    },
    '192.168.0.2' => {
    user   => "george",
        pass   => "tester",
    },
    '192.168.0.3' => {
        user   => "homer",
        pass   => "abc123",
    },
    '192.168.0.6' => {
        user   => "fred",
        pass   => "barney",
    },
);

my %unique;

foreach my $key (keys %HoH) {
    my $user = $HoH{$key}->{'user'};
    $unique{$user} = 1;
}

my $count = keys %unique;
say $count;

--output:--
3
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