put the ip in a variable.
Update:The error was actually in the original regex given not properly matching the names, as far as I can tell, but i think it's much more readable to use a variable name instead of $1, which I think looks confusing and non-intuitive inside of a pattern match.

#!/usr/bin/perl -w

use strict;
my @addrs = (
"192.168.89.1   acmeorp.acme.com",
"192.168.31.3   ftp.acme.com",
"192.168.19.179   [Unknown]"
);
my (@ips,@names);
for (@addrs) {
    push (@ips, $1) if /\b(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})\b/;
    my $ip = $1;
    push (@names, $1) if /\Q$ip\E\s+([\w.]+[^\s]*$|\[Unknown\])\s*?/;
}
print @names;
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