Are you sure parse_line is getting passed what you think it is? The following code behaves as I would expect (regex unchanged):

#!/usr/bin/perl 
use strict;
use warnings;

while (<DATA>) {
    if (/:.+\@.+\s([0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3})/) {
        print $1;
    }
}

__DATA__
random.ircserv.net 378 ircop Blueink :is connecting from *@78.273.80.2
+3 78.273.80.23
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