I don't get that error. Are you certain that what you have is an array of hashes? Here is a one-liner which shows that it works:

perl -Mstrict -Mwarnings -MData::Dumper -e 
'my @array = ( { foo => 1 }, { }, { }, { foo => 2 } ); my
 @elements = grep { exists $_->{foo} } @array; print Dumper
 \@elements'
$VAR1 = [
          {
            'foo' => 1
          },
          {
            'foo' => 2
          }
        ];
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