I'm guessing you need to change this part:

[@{$d[$mapkey]}] if $d[$mapkey]->[1] =~ /$a[$_]/
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That generates an undef when the match fails, and shallow-copies the arrayref $d[$mapkey] when the match succeeds. Sounds like you want something more like (untested):

$d[$mapkey]->[1] =~ /$a[$_]/ ? [@{$d[$mapkey]}] : ()
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to provide an empty list when the match fails. Using ?: in a map is a common idiom.