Nice idea, thanks! I'm using /(?:)/ for now. It's still a bit of a workaround though, since it affects the debug output - you see a lot of "Matching REx "(?:)"" instead of "Matching REx """:

use warnings;
use strict;

'g'=~/g/;
my $regex = 'm{}g';  # user input
my $text = 'x';      # user input
eval qq{
    use re 'debug';
    ''=~/(?:)/;
    print "<\$&>\\n" while \$text =~ $regex;
};

__END__

Compiling REx "(?:)"
Final program:
   1: NOTHING (2)
   2: END (0)
minlen 0 
Compiling REx ""
Final program:
   1: NOTHING (2)
   2: END (0)
minlen 0 
Matching REx "(?:)" against ""
   0 <> <>                   |   0| 1:NOTHING(2)
   0 <> <>                   |   0| 2:END(0)
Match successful!
Matching REx "(?:)" against "x"
   0 <> <x>                  |   0| 1:NOTHING(2)
   0 <> <x>                  |   0| 2:END(0)
Match successful!
<>
Matching REx "(?:)" against "x"
   0 <> <x>                  |   0| 1:NOTHING(2)
   0 <> <x>                  |   0| 2:END(0)
Match possible, but length=0 is smaller than requested=1, failing!
   1 <x> <>                  |   0| 1:NOTHING(2)
   1 <x> <>                  |   0| 2:END(0)
Match successful!
<>
Matching REx "(?:)" against ""
   1 <x> <>                  |   0| 1:NOTHING(2)
   1 <x> <>                  |   0| 2:END(0)
Match possible, but length=0 is smaller than requested=1, failing!
Match failed
Freeing REx: ""
Freeing REx: "(?:)"
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