#!/usr/bin/perl -w
                                                                                
use File::Basename;
use strict;
use warnings;
use diagnostics;
                                                                                
my $pid=$$;
                                                                                
my $name=basename($0);
                                                                                
my $pidfile="/var/tmp/" . $name . "." . $pid;
                                                                                
printf "I'm going to use: %s\n",$pidfile;
                                                                                


##</code><code>##

--$  perl procpid.pl
I'm going to use: /var/tmp/procpid.pl.1974


##</code><code>##

use FileHandle; # Add this with the rest of the uses
:
: # do the filename calculation steps shown above
:
my $fh=new FileHandle("> $pidfile") or die $!;
print $fh "$$"; # Or whatever you want to write
undef $fh; #automatically closes the file


##</code><code>##

$pidfile =~ s/\.pl//;