There are many ways that we can achieve this. If you are using unix system, I hope the following 2 ways will you

use strict;
use warnings;

my $CMD = '/home/blah/my_process';
my $SLEEP_TIME = 1; # check to see if it is running every 5 seconds

while(1)
{
my @pid_array = `ps -ef | grep /home/blah/my_process`;

my $pid_count = scalar(@pid_array);

if ($pid_count == 1)  {
# It isn't running, that 1 is for grep. more than one tells you the pr
+ocess is alive.

system "$CMD"; # run it
}
elsif ($pid_count > 2)  {
# There is more than one instance running.
# Do appropriate stuff
    print "many instances are running";
}

sleep $SLEEP_TIME;
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The above code won't work all the time, but, you can adopt some changes and use the above to find a process is running or not

This is another way. Try the below code

use strict;
use warnings;

if (kill (0, pid))  {
  print "process alive";
}
else  {
  print "process died";
}
[download]

To know about kill function, try google it.