Just to add on to other posts here, if you already have a hash built, keyed on username (such that $hash{$username}->{'Framed-Address'} gets you their IP), you can build a second hash keyed off of IP to simply point to the same data:

# First build us a 'user' key so that our %iphash
# can get the username.

foreach (keys %hash) {
   $hash{$_}->{user} = $_;
}

@iphash{map { $hash{$_}->{'Framed-Address'} }, keys %hash } = values %
+hash;
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Modifications to data under %hash will affect data under %iphash as well, since we're pointing to the same reference.