$; = ':';

my %Hash;

while (<>) {
   chomp $_;
   my ($key, $val) = split(/=/);
   if ($key =~ /:/) {
      my @keyparts = split(/$;/, $key);
      $Hash{ join("$;", @keyparts) } = $val;
        # I'm not sure how to get perl to not scalar-ize it in this case
        # But it's probably better to reduce cargo-cultism ;)
   } else {
      $Hash{$key} = $val;
   }
}

foreach (sort keys %Hash) {
   print "$_    \t=> " . $Hash{$_} . "\n";
}

print $Hash{'a1','b1','c1'} . "\n"