Unless @found has pre-existing contents, you can drop the push as well.

my @a = ( 1, 3, 5, 7, 9, 11 );
my @hash{ 1 .. 10 } = 'a' .. 'j';

my @found = map{ exists $hash{$_} ? "$_$hash{$_}" : () } @a;

print "@found";
1a 3c 5e 7g 9i
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