perl special charecter

$_ 

This is a special array for the subroutine duration.U can access it by use $_[<index>]. So the first parmater will be $_[0].

The @_ variable is private to the subroutine, @_ is always the parameter list for the current subroutine invocation

$PROCESS_ID

$$

read a file line by line

#!/usr/bin/perl
use strict;
use warnings;

my $dir ="./tmp";

open (FH, "<$dir/txn-history.txt") or die "Couldn't open $dir/txn-history.txt, $!"."
";
while(<FH>) {
print "$_";
}

perlref

If you put a  in front of a variable, you get a reference to that variable.

perlobject

perl's subroutine first argument is itself or class/package name.