package My_Enc_Dec;
use strict;
use warnings;
use base 'Exporter';
our @EXPORT_OK;

{
  my $passphrase;
  BEGIN { $passphrase = 'choose something better'; }
  use Crypt::RC4;
  use MIME::Base64;

  push @EXPORT_OK, 'enc';
  sub enc {
    ( my $ret = encode_base64( RC4( $passphrase, shift ))) =~ s/==\n$//;
    return $ret;
  }

  push @EXPORT_OK, 'dec';
  sub dec {
    return RC4( $passphrase, decode_base64( shift ));
  }
}

1;
__END__
# ............................................................
# link-generating code
use strict;
use warnings;
use My_Enc_Dec 'enc';
my $url  = 'http://www.domain.com/cgi-bin/fetch.pl';
my $exp  = enc( time + 60*60*24 );
my $link = "$url?$exp";

# ............................................................
#!perl
# fetch.pl
use strict;
use warnings;
use CGI;
use My_Enc_Dec 'dec';

my $exp = dec( param( 'keywords' ) );
time <= $exp or have_a_cow();