When reading data from files, Perl allows you to obtain the name of the
file with the $ARGV variable, and the line number with the $. variable.
This makes it easy to obtain information on which file (and which line of
the file) is under the gun at any given moment. Consider the following
example, which demonstrates:

  #!/usr/bin/perl

# read a file and print its contents

  while (<>)

  {

  # for each line, print file name, line number and contents

  print $ARGV, ",", $., ": ";

  print;

  }

  

Here's an example of the output:

multiply.pl,1: #!/usr/bin/perl
multiply.pl,2:
multiply.pl,3: # multiply two numbers
multiply.pl,4: sub multiply()
multiply.pl,5: {
multiply.pl,6: my ($a, $b) = @_;
multiply.pl,7: return $a * $b;
multiply.pl,8: }
multiply.pl,9:
multiply.pl,10: # set range for multiplication table
multiply.pl,11: @values = (1..10);
multiply.pl,12:
multiply.pl,13: # get number from command-line
multiply.pl,14: foreach $value (@values)
multiply.pl,15: {
multiply.pl,16: print "$ARGV[0] x $value = " . &multiply($ARGV[0],
$value) . "\n";
multiply.pl,17: }

Note, however, that the line number returned by $. does not automatically
reset itself when used with multiple files, but rather keeps incrementing.
In order to have the variable reset to 0 every time a new file is opened,
you need to explicitly close() the previous file before opening a new one.

The $0 variable returns the file name of the currently-running Perl script,
as illustrated below:

  #!/usr/bin/perl

# print filename

  print "My name is $0";

  

Here's the output:

My name is /tmp/temperature.pl

The $$ variable returns the process ID of the currently-running Perl
process, as below:

  #!/usr/bin/perl

# print process ID

  print "This script is owned by process ID $$. Collect them all!";

  

Here's the output:

This script is owned by process ID 2209. Collect them all!

Finally, the special variable $] always contains information on the Perl
version you are currently running. So, for example, the program

  #!/usr/bin/perl

# print Perl version

  print "Running Perl $]";

  

might return something like this:

Running Perl 5.008

This, coupled with the $0 and $$ variables explained earlier, can come in
handy when debugging misbehaving Perl programs,

  #!/usr/bin/perl

# check for error

  if ($error == 1)

  {

  # write to error log with script name, PID and Perl version

  open (FILE, ">>/tmp/error.log");

  print FILE "Error in $0 (perl $] running as PID $$\n";

  close (FILE);

  }

  

or even to perform version checks to ensure that your code only works with
a specific Perl version.

  #!/usr/bin/perl

# check version

  # display appropriate message

  if ($] < 5.0) 

  {

  die("You need a more recent version of Perl to run this program");

  }

  else

  {

  print "This is Perl 5 or better";

  }