The following module makes it easy to only let one copy of a program run at a time. At the top of your script add: 
    use My::ThereCanBeOnlyOne '/tmp/myprogram.pid';

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You can also specify LockFile::Simple options to change the behavior as in: 
   use My::ThereCanBeOnlyOne '/tmp/myprogram.pid',
                             -hold  => 0,
                             -stale => 1,
                             -max   => 1;

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It is recommended you chose a lockfile outside of /tmp as that would allow a denial of service attack from another user on the same machine. 
package My::ThereCanBeOnlyOne;
use strict;
use LockFile::Simple;

my $Lock;

sub import {
  my $self     = shift;
  my $lockfile = shift || die "please provide lockfile";
  my %args     = @_;

  my $locker = LockFile::Simple->make(-autoclean => 0, %args);
  $Lock = $locker->lock($lockfile, '%f')
    and return 1;

  open(LOCKFILE, "< $lockfile")
    or die __PACKAGE__."Unable to open $lockfile: $!";
  my $other_pid = <LOCKFILE>;
  close(LOCKFILE);
  chomp($other_pid);
  die __PACKAGE__.": $lockfile: $other_pid still running. $$ exiting\n
+";
}

END {
  $Lock->release();
}

1;

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In reply to Only let one copy of program run at a time by esh

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