You can use the convert_blessed option. To add a TO_JSON method to regexes, just define it in the ref qr() namespace, i.e. Regexp : 
#! /usr/bin/perl
use warnings;
use strict;
use JSON::XS;

my $json = JSON::XS->new->convert_blessed;
sub Regexp::TO_JSON { "" . shift }

my $re = qr/^-{1,}\z/;
print $json->encode([$re]), "\n";

[download]

Update:

You can even localize the change, so that regexes behave normally in other parts of the code: 

{   no strict 'refs';
    local *{'Regexp::TO_JSON'} = sub { "" . shift };
    print $json->encode($re), "\n";
}

[download]

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

[download]

In reply to Re: Freezing a regular expression qr() by choroba
in thread Freezing a regular expression qr() by Anonymous Monk

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