You want to know about @ARGV:

use strict;
use warnings;

my ( $gate1, $gate2 );

if ( @ARGV == 2 ) {
    ( $gate1, $gate2 ) = @ARGV;
}
else {
    die "Usage:  $0  gate1_name  gate2_name\n";
}

# no need to chomp @ARGV values...
[download]

(update: changed to declare the "gate" variables outside the "if", so they would be available for use outside the if block)