Read file if the command line argument is specified, or read from standard input.

input.pl

#! /usr/bin/perl

use strict;
use warnings;
use Getopt::Long;
use Pod::Usage;

# global variables
my $help = 0;
my $man = 0;

GetOptions(
  'help|h' => \$help,
  'man' => \$man
) or pod2usage();

pod2usage(1) if $help;
pod2usage(-exitstatus => 0, -verbose => 2) if $man;

&main; exit;

sub main {
  my $handle = 'STDIN' unless $ARGV[0];
  (open $handle, "<", $ARGV[0] or die "Cannot open $ARGV[0], $!") if $ARGV[0];

  while (<$handle>) {
    print;
  }
}

# functions
__END__

=head1 NAME

input.pl - Write short description of this script here

=head1 SYNOPSIS

 input.pl [options] [file ...]

 Options:
   -help, h         brief help message
   -man             full documentation

=head1 OPTIONS

=over 2

=item B<-help, h>

Print a brief help message and exits.

=item B<-man>

Prints the manual page and exits.

=back

=head1 DESCRIPTION

B<This program> will read the given input file(s) and do something
useful with the contents thereof.

=cut