Yes, the parens are being treated as regexp metacharacters. You can escape it outside the pattern using quotemeta($value), or within the pattern using \Q...\E:

However I'd normally rather do this as a hash lookup - faster, and it's already an equality check:

  my %enum_values = map +($_ => 1), @{ $constraints->{enum} };
  if (keys(%enum_values) && !$enum_values{$value}) {
    my $enum_values_string = join ', ', sort keys %enum_values;
    ...
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Another option is using the List::Util function 'none' (or 'any' for the inverse):

  use List::Util qw{ none };
  if (none { $_ eq $value } @$enum_values) {
    ...
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