Maybe I don't understand the question, but it appears to me that you want the list of REQUIRED_FIELDS minus the ones that are in your $a which is a reference to an array?

I personally would use a different return value, either a list or an array reference as the output rather than pervert (grossly modify) the input to such an extent that it no longer means what it did as the input.

I mean if the input is "here's what I have" and the output is "here's what you didn't have" - those are two very different things.

It looks to me like the output of verify() is of a different "type" or "unit" than the input. I would return a list like I did, or a reference to an array of the "new type".

#!/usr/bin/perl -w
use strict;

use constant REQUIRED_FIELDS => qw/zero one two three four five six se
+ven/;

my $a_ref = [qw/three one two/];

my @missing_fields = verify($a_ref);
print "missing fields: @missing_fields\n";

sub verify
{
    my $a_ref = shift;
    
    my %seen = map{$_ => 1}@$a_ref;
    
    return (grep{!$seen{$_}}REQUIRED_FIELDS);
}

__END__ 
Prints:
missing fields: zero four five six seven
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