"My rationale (which might be an oxymoron) was that if I didn't have an Accept value (one wasn't provided). the unless would ensure an "always valid" -- see; defined value. No?"

No! And that's very easy to test:

#!/usr/bin/env perl -l

use strict;
use warnings;

my %test_hash;

if ($test_hash{unset_key} =~ /x/) {
    print 'true';
}
else {
    print 'false';
}

unless ($test_hash{unset_key} =~ /x/) {
    print 'false';
}
else {
    print 'true';
}
[download]

Output:

Use of uninitialized value $test_hash{"unset_key"} in pattern match (m
+//) at ./pm_example.pl line 8.
false
Use of uninitialized value $test_hash{"unset_key"} in pattern match (m
+//) at ./pm_example.pl line 15.
false
[download]

Perhaps you want exists and/or defined.