As Tanktalus said in Re: given-when construct unexpected bahavior wit arrays, this seems not natural.

Here, something doing what you want (I also change first grep with "eq"), but I feel bad with that trick :-)

#!/usr/bin/env perl

use v5.14;

my @array = (0, 1000..10_000, 'abcd');

for my $element (qw/a ab abc 100 1000 10000 abcd/) {
   say "With grep:";
   if (grep {$element eq $_} @array) {
      say "Element $element is there";
   } else {
      say "Element $element is not there";
   }
   
   say "With given-when:";
   given ("_$element") {
      when ([map {"_$_"} @array]) {
         say "Element $element is there";
      }
      default {
         say "Element $element is not there";
      }
   }
   say "";
}
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