%hoh = map { $_, { map { $_, 1 } @{ $hol{$_}} } } keys %hol;


##</code><code>##

#!/usr/bin/perl -w

use strict;

my %hol = (
    first => [ 1, 2, 3 ],
    second => [ 2, 4, 6 ] );

my %hoh;
%hoh = map { $_, { map { $_, 1 } @{ $hol{$_}} } } keys %hol;

walk_hash(\%hoh);

sub walk_hash {
    my $h_ref = shift;
    my $indent = shift;
    $indent ||= 0;
    foreach my $key (keys %$h_ref) {
        print "\t" x $indent, "$key => ";
        my $value = $h_ref->{$key};
        if (ref($value) eq 'HASH') {
            print "\n";
            walk_hash($value, $indent + 1);
        } else {
            print "\t" x $indent, "$value\n";
        }
    }
}