"John Doe <john@example.org>" (minus the quotes) is a valid email address, but you'll have problems if $recipients contains that.

And of course, there's the malicious who might find a way of getting "| rm -rf /" (minus the quotes) into $host or $recipients.

The multiple argument form of various commands gets around that problem.

use IPC::Open3 qw( );

open(my $mailer_fh, '|-', '/bin/mail', '-s', $hostn, $recipients)
   or die("Cannot execute mail command: $!\n");

open(my $msg_fh, '<', $disk_changes)
   or die("Cannot execute message file: $!\n");

print $mailer_fh $_ while <$msg_fh>;

close($mailer_fh)
   or die("Error sending mail message: $!\n");

$? == 0
   or die("Error sending mail message: $?\n");
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