Unless (thing)

Perl has an unless statement (and an unless modifier), which work exactly like the corresponding ifs, but negate their condition.

So you can say

die "Usage: $0 [-x] file1 file2\n"
  unless @ARGV >=2 and @ARGV <= 3;

You can also say

sub factorial {
  my $n = shift;
  unless ($n <= 1) {
    $n*factorial($n-1);
  }
  else {
    1;
  }
}

but you probably shouldn't.