I'm still new to programming and have been working through Lerning Perl and LPORM in conjunction with the perldoc. While reading an article I came across about how credit card numbers work, I figured it was a decent opportunity to practice what I've been learning. I know this is basic code, but I was hoping that in offering it up, the monks would review it and tell me if I'm striking out syntactically/semantically and/or demonstrating ignorance of generally accepted standards/conventions. The code runs, and does what it's supposed to. I plan on modifying it later to include validation for 13,14, and 15 digit credit card numbers as well as possibly reading a list of numbers from a text file and writing only valid numbers to a new file. I wanted to wait on expanding the program until I was sure that I wasn't completely off track. 
#!/usr/perl/bin
#Simple program to check validity of 16 digit credit card numbers

use strict;
use warnings;

sub checkNumber {
   my $in = shift;
   my @cardNum = @{$in};
   my $i = $#cardNum - 1; 

   while ($i >= 0) {
      $cardNum[$i] = $cardNum[$i] * 2;
      if ($cardNum[$i] >= 10) { $cardNum[$i] -= 9; }
      $i -= 2;
   }
   
   my $total = 0;
   $total += $_ foreach (@cardNum);
   if ($total % 10 == 0) { 
      return 1;
   } else {
      return 0;
   }
}

sub cardInfo {
   my $cardNum = shift;
   my $issuer; 
   if ($cardNum =~ /^4/) { $issuer = "VISA" }
   if ($cardNum =~ /^5/) { $issuer = "MasterCard" }
   if ($cardNum =~ /^6/) { $issuer = "Discover" }
   print "\n   Card Number: $cardNum\n";
   print "   Issuer:  ",  $issuer,"\n\n";
}

print "Enter Number:   ";
chomp(my $cardNum = <STDIN>);

if ($cardNum =~ /^[0-9]+$/) {
   my @number = split(//,$cardNum);
   if ($#number == 15) {
      if (checkNumber(\@number)) {
         cardInfo($cardNum);
      } else { 
         print "\nInvalid Card Number\n";
      }
   } else {
      print "\n16 digit cards only for now.\n";
   }
} else { 
   print "\nInvalid Input.\n";
}

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In reply to Code Review: Simple Credit Card Number Validation by marquezc329

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