If you already have the very useful def in your toolbag:

  sub def {
      $_[0]= def($_[1],"") if ! defined $_[0];
      return $_[0];
  }
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then you can do this as:

  $ext{index} = 'pod';
  for my $pod (  qw( HostCycle Instinet index )  ) {
      system "pod2html $pod" . def($ext{$pod},".pm") . " > $pod.html";
  }
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def (short for "define" which would be too easy to confuse with "defined") is useful for avoiding warnings. You can use it like || as above or like ||=. It differs from these two operators in that it triggers on definedness rather than truthness.

Note that def is so useful that even def uses it. ;)

        - tye (but my friends call me "Tye")