The problem is that the expression $_->{key} => $_->{value} if($_->{key} =~/^a\d+/) will still return something if $_->{key} =~/^a\d+/ doesn't match.    You should try it like this:

my %ret = map $_->{key} =~ /^a\d/ ? ( $_->{ key } => $_->{ value } ) :
+ (), @$data1;
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