Yep, doing map {"_$_"} @array before each test, in given-when, is not good.

But grep works too much because if the first array element matches, you don't need to look others.

This is probably the fastest solution :

sub is_in_array2 {
   my $element = shift;
   ($element eq $_) && return 1 for @_;
}
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update: You should also try with an hash %is_in_array