OK, let's forget my original problem because it is already solved (Eily pointed out that before I convert a string into Regexp object, I can just print it first, and in my case it's true, all I have is a string). But sometimes I do have a Regexp object, for example in my other use-case the validation specification already contains a Regexp object, for example:

$foo_spec = {
    args => {
        arg1 => {required=>1, match=>qr/\A\w+\z/},
        arg2 => {required=>1},
    },
};
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Now the validator code generator must generate this string:

defined($arg1) or die "arg1 must be defined";
$arg1 =~ qr/\A\w+\z/ or die "arg1 does not match regex";
defined($arg2) or die "arg2 must be defined";

The spec can be written by others.

Of course, I can require the 'match' key to have a value of string and not Regexp object, so this avoids the problem.

But I'm interested in knowing whether a newer Perl (5.14+) can stringify a Regexp object in a way that is compatible with older Perls.