I believe

my $is_valid = utf8::decode($_);
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is cheaper than

use Encode qw( decode );
my $is_valid = eval '$_ = decode("utf-8", $_, FB_CROAK); 1';
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It's definitely simpler (and you don't even need to load any modules!)

Note that "utf8" is not the same thing "utf-8". "utf8" is the name of Perl's internal encoding. It differs from "utf-8". You definitely want to use "utf-8" when validating (if not always).

I also fixed the bug where decoding the string "0" would be considered a validation error.