Since "success" and "failure" seem to be pretty arbitrary, I might consider reversing the role of your function here:

my $failure = is_taken($domain);

print "Failed: $failure\n" if $failure;
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In the more general case, some modules tend to use $@ to store error messages, or a package-specific variable (say, $Lookup::errstr) containing the message:

unless (lookup($domain)) {
   print $@ ? "$domain lookup failure: $@\n" 
            : "$domain is available!\n";
}
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