There is another problem with this bit of logic: If you want a default for the first argument ($based_on), you still have to specify it:

  mycode.pl tp 5
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if you do not, the script will assume the value you passed for $top is your $based_on, eg:

  mycode.pl 5
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So this logic model does not make sense. You can differentiate between the arguments for example by putting a '-' in front of the $based_on argument, or if the argument passed to $top is always numeric, you can use that as well. Best is to ensure your arguments are exactly right:

      1 #!/usr/bin/perl -w
      2 
      3 use strict;
      4 
      5 my $based_on = ($ARGV[0] && ($ARGV[0] =~/tp|sn/)) ? shift : 't
+p';
      6 my $top = ($ARGV[0] && ($ARGV[0] =~ /[1-5]/)) ? shift : 1;
      7 scalar(@ARGV) && die "Arguments not parsed: " . join(':', @ARG
+V) . "\n";
      8 
      9 print "$based_on $top\n";
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-Reenen