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qw( A A A A )
qw( B B )
qw( C C C )
qw( D )

qw( A C B A D C A B C A ) # I think...

# Sublists generated by an iterator.
my @p1 = ( 'A'  , undef, undef,   'A', undef, undef,   'A', undef, und
+ef,   'A' );
...

print "@solution\n";
# Produces A C B A D C A B A C