Because you've already shifted out ARGV[0] so ARGV[1] is now ARGV[0].

I'd be inclined to:

use strict;
use warnings;

my ($based_on, $top) = @ARGV;
$based_on ||= 'tp';
$top ||= 1;
print "$based_on $top\n";
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which is fine so long as $top can't == 0