$line =~ s/.*\]: //;

##</code><code>##

$line =~ s/.*?\]: //; # note the ? following .*

##</code><code>##

if  ($line =~ /\[(\d+\.\d+\.\d+\.\d+)\].*Relay access denied/) {
    my $ip = $1;
    print "$ip\n";
}