{
    my %orig;
    sub re_db_on (\$) {
        my ($r) = @_;
        die "not a regex" unless ref $$r eq 'Regexp';
        return if $orig{$r};
        $orig{$r} = $$r;
        # note this depends on the proper stringification of the regex
        $$r = eval qq{ use re qw/Debug EXECUTE/; qr/$$r/ };
    }
    sub re_db_off (\$) {
        my ($r) = @_;
        return unless $orig{$r};
        $$r = delete $orig{$r};
    }
}

my $ra = qr/a/;
my $rb = qr/b/;
my $rc = qr/c/;
my $str = "abc";
$,=", "; $\="\n";
print $str=~$ra, $str=~$rb, $str=~$rc;
re_db_on $rb;
print $str=~$ra, $str=~$rb, $str=~$rc;
re_db_off $rb;
print $str=~$ra, $str=~$rb, $str=~$rc;