Map in void context - bad. just use a for loop:

@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: $!";
}
[download]

I've just made it longer, oh well...