Please have a look at my post here about the security issues with calling external commands. I really hope you've sanitized the variables you are interpolating into that shell command.

I really don't want to use packages for such a simple operation.

You will be doing yourself a huge favor by using a module like Email::Sender and the other Email::* modules by rjbs. Yes, even you can use CPAN! Example code.

But if you really, really want to do it in pure Perl, then at least use a piped open instead of qx + cat:

use warnings;
use strict;
use 5.008;
my @cmd = ('mailx','-r',$EMAIL_ADMIN,
    '-s',"Exchange Backup Status $rptdate",$SEND_TO);
die '@cmd must have more than one element' unless @cmd>1;
open my $fh, '|-', @cmd or die $!;
print $fh $mail_body;
close $fh or die $! ? $! : $?;
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