Try

#!/usr/bin/perl
use strict;
use warnings;
use MIME::Base64;
use IO::File;
use File::Basename 'basename';

my $from    = '';
my $to      = '';
my $subject = "Test Mail";
my $mailbody = "This is a test";

my $attachment = "test.zip";
my $basename   = basename($attachment);

open(MAIL, "|/usr/sbin/sendmail -t");
print MAIL << "EOF";
From: $from
To: $to
Subject: $subject
Content-Type: multipart/mixed; boundary=frontier
--frontier
Content-Type: text/plain; charset=us-ascii

$mailbody

--frontier
Content-Disposition: attachment; filename=$basename
Content-Transfer-Encoding: base64
Content-Type: application/octet-stream; name=$attachment

EOF

open FILE, '<', $attachment or die "Cannot open $attachment: $!";
binmode FILE;
while (read(FILE, my $buf, 60*57)) {
  print MAIL encode_base64($buf);
}
close FILE;
print MAIL "\n--frontier\n";
close MAIL;
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