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use Data::Dumper;
use List::Util qw(first);
...

say Dumper([grep { $_->{"aaa"} < 4 } @x]->[0]);
say Dumper((first { $_->{"aaa"} < 4 } @x));

$VAR1 = undef;

$VAR1 = undef;

say Dumper([grep { $_->{"aaa"} < 4 } @x]->[0]->{"aaa"});
say Dumper((first { $_->{"aaa"} < 4 } @x)->{"aaa"});

$VAR1 = undef;

Can't use an undefined value as a HASH reference at test.pl line 7.