use strict;
use warnings;

my %components = (
    COMP1 => {
        eth1    => "10.172.9.21/24",
        eth1_gw => "10.172.1.1",
        eth2    => "10.172.10.21/24",
    },
    COMP2 => {
        eth1 => "10.172.9.22/24",
        eth2 => "10.172.11.21/24",
    },
    COMP3 => {
        eth1 => "10.172.11.21/24",
        eth2 => "10.172.13.21/24",
        eth3 => "10.173.2.98/24",
    },
);

my %addresses;
foreach my $comp ( keys %components ) {
    foreach my $eth ( keys %{$components{$comp}} ) {
        push(@{$addresses{$components{$comp}{$eth}}}, "$comp:$eth");
    }
}

foreach my $address ( keys %addresses ) {
    print "$address: appears on " . join (', ', @{$addresses{$address}}) . "\n";
}