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sub isprime {
    my $n=shift;
...
    $n%$_ or return 0 for (2..sqrt $n);
    1;
}

sub isprime {
    (1 x shift) !~ /^1?$|^(11+?)\1+$/;
}

sub nearest_prime {
    my $n=my $m=shift;
...
        return $m if isprime --$m;
    }
}