This is the code from my "email parse script". I need to extend this script with a DKIM check.

#! /usr/bin/perl

use strict;
use warnings;
use Email::Simple;
use DBI;
use File::Copy;
use Date::Parse;
use DateTime;
use Path::Tiny qw(path);

[..]

open (MESSAGE, "< $ARGV[0]") || die "Couldn't open email $ARGV[0]\n";
       undef $/;
$raw_email = <>;
close MESSAGE;

my $mail            = Email::Simple->new($raw_email);
my $from_header     = $mail->header("From");
my $to_header       = $mail->header("To");
my $date_header     = $mail->header("Date");
my $aol_header      = $mail->header("Authentication-Results");

[...]
[download]

This is the code from my "email parse script"

So that section is not relevant to your test script. If we omit it we can end up with a working version such as:

use strict;
use warnings;
use Test::More;
use Mail::DKIM::Verifier;

my @good = qw/good1.txt good2.txt/;
my @bad  = qw/bad1.txt bad2.txt/;
plan tests => @good + @bad;

for my $goodfile (@good) {
    my $dkim = Mail::DKIM::Verifier->new();
    open (my $gfh, '<', $goodfile) or die "Error opening $goodfile: $!
+";
    $dkim->load ($gfh);
    is ($dkim->result_detail, 'pass', $goodfile);
    close $gfh;
}
for my $badfile (@bad) {
    my $dkim = Mail::DKIM::Verifier->new();
    open (my $bfh, '<', $badfile) or die "Error opening $badfile: $!";
    $dkim->load ($bfh);
    isnt ($dkim->result, 'pass', $badfile);
    close $bfh;
}
[download]

Obviously you can change the filenames in the @good and @bad arrays until you have what you want. This script works fine for me testing on good and bad data.