my %hash = (
    keya => [a1, b1, [ qw(c1a c1b) ] ],
    keyb => [a2, b2, [ qw(c2a c2b) ] ],
);
foreach my $key (sort keys %hash){
  print $hash{$key}[0];
  print $hash{$key}[1];
  print $hash{$key}[2][0];
  print $hash{$key}[2][1];
}
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note: don't use a comma inside qw() - it expects just spaces.