I don't believe you are seeing a leak. I have to express it this way because I don't have *nix to test on; but running your code on my system, I see memory growth, but not a leak.

To confirm my findings, tweak your code so:

#!/home/alerting/perl5/perlbrew/perls/perl-5.20.2/bin/perl

use warnings;
use IO::Handle;
use threads stack_size => 64*1024;
use strict;
use DBI;
use Email::MIME;
use Email::Sender::Simple qw(sendmail);

while (<STDIN>)
{
     threads->create(\&thread)->detach;
     sleep 5;
}

sub thread
{
        print "Thread started!\n";
        sleep 4;
}
[download]

Essentially what that does is limit your program to using only child thread at a time. (Not useful in the real world, but good for testing.).

Run that and hold the enter key down while monitoring the memory usage and you should see an initial jump from circa 7MB to around 13MB, and then the memory usage should stay steady no matter how long you hold the key down for. Come back and let me know if that is true; and then I'll try and explain what you are seeing.