This script nicely demonstrates a bad use of map - if you are discarding the output of map then you should use for/foreach.

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));
[download]

For real I'd put an or die after the system though ;-)

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

It works! (The error messages are from pod2html.)