use strict;
use warnings;

# take the files as command line arguments
# @ARGV contains all arguments passed in
my $first_file  = shift (@ARGV);
my $second_file = shift (@ARGV);

# double check that the file exists
# that they can be read
# and that they are text

unless ((-e $first_file) and
        (-r $first_file) and
        (-T $first_file) and
        (-e $second_file) and
        (-r $second_file) and
        (-T $second_file)) {
  die ("Can't open one of the files!");
}

open("FIRST", "< $first_file")
  or die ("Can't open $first_file because $!");
open("SECOND", "< $second_file")
  or die ("Can't open $second_file because $!");

my $linenumber = 0;
while (1) {
  # we can use an infinite loop because
  # we have a last in here...
  # I could have compressed everything into the
  # conditional () but that makes it harder
  # to understand what is going on
  my $line1 = <FIRST>;
  my $line2 = <SECOND>;
  last unless ($line1 and $line2);
  $linenumber++;
  if ($line1 eq $line2) {
    print "$linenumber matches on each file!";
    # note we used eq
    # == is for numerical values
    # eq is for strings
  }
}

print "We processed $linenumber lines\n";