As a minor nit, you can do away with this $1 stuff and assign directly to $ip like this: 
if  (my ($ip) = $line =~ /\[(\d+\.\d+\.\d+\.\d+)\].*Relay access denie
+d/) {
    print "$ip\n";
}

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In reply to Re^2: Regex substitute matches second match first? by Marshall
in thread Regex substitute matches second match first? by Linicks

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