Nobody ° offered a postfix for, so for completeness

    
 $hash{ delete $_->{id_1} }{ delete $_->{id_2} } = $_
     for @ary_of_hrefs ;
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Untested, but most code was copied from marioroy's post :)

Personally I try to avoid map statements in void context

For clarity I strongly recommend commenting this with a sample of the data before and after.

update

°) Sorry Eily did, tldr my apologies :)