What are you trying to do with:

Dumper %RegHash = $error;
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It looks like you are assigning a scalar to a hash to me and that is not going to work.

If I edit your while loop and environs a little to give:

            # Dumper %RegHash = $error;
            # while (@set = 2)
                {
                @set = each %RegHash or die "failure to read registry:
+ $! \nlast contents of Registry Hash = $error \n";
                last if @set != 2;
                 if ($set[0] eq "/"){
                     print "$val = $set[0]\n";
                     $watchcode = 1;
                }
                 
                next; # added
            }
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then I get output tht looks like:

.3/ = /
.386/ = /
.A51/ = /
.ac3/ = /
.aca/ = /
.ace/ = /
.acf/ = /
...
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Update: The following is probably closer to what you actually want:

#!/usr/bin/perl
use warnings;
use strict;
use Win32::TieRegistry (Delimiter => "/");

exit if defined($ARGV[0]); # Why btw?

foreach my $class (sort keys %{ $Registry->{"Classes/"} }){
    next unless $class =~ /^[.]/;
    my %RegHash = %{ $Registry->{"Classes/$class"} };

    print "$class = $RegHash{'/'}\n" if exists $RegHash{'/'};
}
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Prints:

.3/ = 3_auto_file
.323/ = h323file
.386/ = vxdfile
.3g2/ = QuickTime.3g2
.3gp/ = QuickTime.3gp
.3gp2/ = QuickTime.3gp2
...
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Update: Doh! replaced inner loop per ikegami

Note that missing associations is not an error so no warning or dies. The if exists modifier could be changed to a full blown if and note the missing association in the else part.