I would write this

grep { $h1{$_}=1 } @{$all[0]};
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like this

grep { $h1{$_}=1; undef; } @{$all[0]};
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in order not to get all my elements from @{$all[0]} as a return value of grep.

I even might write:

@h1{@{$all[0]}}=(1) x @{$all[0]};
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