You can tidy the ternary operator up too. I'd write something like (untested but optimised for brevity :-)

  $extension{index} = 'pod';
  system "pod2html $_." . ($extension{$_} || 'pm') . " > $_.html"
      for (qw(HostCycle Instinet index));
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Update: I did test it, and I got this result

$ perl -
[fx: cuts and pastes the code above]
Can't open HostCycle.pm: No such file or directory
Can't open Instinet.pm: No such file or directory
Can't open index.pod: No such file or directory

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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  $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. ;)

my @pod = qw(HostCycle Instinet index);
my %ext = ( index => 'pod' );
system "pod2html $_.".($ext{$_}?$ext{$_}:'pm')." > $_.html" foreach @p
+od;
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--
Casey
   I am a superhero.

@pod = qw(HostCycle Instinet index);
$extension{index} = '.pod';
for (@pod) {
 # Assuming there's no '0' or blank extensions
 $infile_extension = $extension{$_} || '.pm';
 # Otherwise use this
 #$infile_extension = exists $extension{$_} ? $extension{$_} : '.pm';
 # Check status ?
 system "pod2html $_$infile_extension > $_.html" and die "pod2html fai
+led: $!";
}
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