My contribution, cos it had to be done:^). Probably not the most efficient solution, but simple.

#! perl -w
my @domains = qw(foo.com weirdext.za bar.uk.com blah.co.uk perl.pl zzz
+z.co.uk);

my @sorted = map{ join '.',  reverse split( /\|/,$_ ) }
              sort 
              map {join '|', reverse split( /\./,$_,2) }  @domains;

{ local $"="\n"; print "@sorted"; }
__END__
# Output
C:\test>193114
blah.co.uk
zzzz.co.uk
foo.com
perl.pl
bar.uk.com
weirdext.za
C:\test>
[download]

I know that the result is slightly different from your 'desired output' example, but I thought about this for a long time, and whilst I'm probably wrong as noone else has mentioned it, I can see no criteria by which bar.uk.com could be sort in the position you have it?

If its grouped with foo.com, because they both have a .com extension, then bar.uk sorts before foo.

If its after foo.com because .uk.com is lexically higher that .com, then .uk.com is also higher than .pl, which is what I think that you are asking for.