#!/usr/bin/
#-- try: #!/usr/bin/perl
#-- or:  #!/usr/bin/env perl

#program to generate 6 Million unique codes, Oct 15, 2011.

#-- 'use strict;'   - strongly recommended
#-- 'use warnings;' - strongly recommended


open(MYOUTFILE, ">codesSixMil.txt"); #open for write, overwrite (>)
#-- 1.) 3-argument-open() recommended
#-- 2.) filehandle could be e.g. $codesfile instead of MYOUTFILE, but it's
#       okay for a small script.
#-- 3.) check if open fails:
#       'open( MYOUTFILE, '>', "codesSixMil.txt") or die "cannot open codesSixMil.txt - $!";'
#-- 4.) maybe use a variable for filename: 'my $code_filename = "codesSixMil.txt"

#-- 'use strict;' will tell you to rewrite the following lines
#    as 'my $count = 0;' etc.
$count  = 0; #keep track of how many codes generated
$count1 = 0; #keep track of how many codes duplicated
#-- bad variable names, esp. $count1 --> e.g. $codes_generated, $codes_unique

$desiredQuantity = 6000001;
#-- 'off by one' smell - if you want exactly 6M then write 6000000 and adjust loop
#-- (or more readable: 6_000_000)

@arrayOfResults =();
#-- bad variable name: suggestion: @unique_codes or @raw_codes or ...

sub generate_random_string {
   my $stringsize = shift;
   my @alphanumeric = ('a'..'z', 3,4,6,7,9);

   my $randstring = join '', (map { $alphanumeric[rand@alphanumeric] } @alphanumeric)[0 .. $stringsize];
#-- maybe:
#  my $randstring = join '', (map { $alphanumeric[rand@alphanumeric] } (1..$stringsize);

   return $randstring;
}

for ($i=0;$i<$desiredQuantity;$i++) {
  #-- C-style loop. More Perlish: 'for (1..$desiredQuantity) {'
   $returnvalue = generate_random_string(10);
  #-- bad name. Suggestion: $code_key or the like...
  #-- maybe use a $code_length variable instead of '10'?
   push(@arrayOfResults, $returnvalue);
   $count++; #-- not very useful: after the loop: '$count == $desiredQuantity;'
             #-- Can be replaced by $desiredQuantitity (maybe).
}

#remove any potential duplicates
@unique = grep { ++$count{$_} < 2 } @arrayOfResults;
#-- waste of memory, but should work

#loop through array to output each code on its own line
#with a carriage return as required.

for ($i=0;$i<$desiredQuantity;$i++) {
#-- needless use of C-style loop here
#-- if you have duplicates, @unique < @arrayOfResults, so
#-- replace $desiredQuantity by $#unique - or better
#-- rewrite the loop in 'for my $code ( @unique ) {' style
    print @unique[$i], "\n";  #-- for debugging? Increases runtime.
    print MYOUTFILE @unique[$i], "\r\n";
    $count1++; #-- see $count above
}

#do some rudimentary checking and output result.
print "\n---Code generation report--- \n\n";
print "First array: $count \n";
print "     Unique array: $count1 \n\n";
print "---There are ", $count-$count1, " code duplicates in this listing of ", $desiredQuantity, " ---\n\n\n";

#-- Your current implementation will always tell you: $count1 == $count == $desiredQuantity
#-- even if @unique < @arrayOfResults giving you the false impression to have $count unique codes

#*** Close the file ***
close(MYOUTFILE);