"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");