How to pass command-line arguments to a Perl program?

Depends on what you want to do. If you want to use the two arguments as input files, you can just pass them in and then use <> to read their contents.

If they have a different meaning, you can use GetOpt::Std and GetOpt::Long to process them easily. GetOpt::Std supports only single-character switches and GetOpt::Long is much more flexible. From GetOpt::Long:

use Getopt::Long;
my $data   = "file.dat";
my $length = 24;
my $verbose;
$result = GetOptions ("length=i" => \$length,    # numeric
                    "file=s"   => \$data,      # string
                    "verbose"  => \$verbose);  # flag

Alternatively, @ARGV is a special variable that contains all the command line arguments. $ARGV[0] is the first (ie. "string1" in your case) and $ARGV[1] is the second argument. You don't need a special module to access @ARGV.