This is one of the magic edge cases which confuse me in Perl. :(

They are nice for short scripts but a source of errors in larger apps, because they break orthogonality.

Anyway I'm confused about the result you are expecting... the empty pattern would always match for every position

use strict;
use warnings;
use feature 'say';


my $run=0;

sub test {
    $run++;
    my $regex = 'm{}g';                 # user input
    my $text = 'xx';                    # user input
    print "--- run $run\n";
    pos($text)=0;
    eval <<"__CODE__";
        say pos(\$text),"<\$&>" while \$text =~ $regex;
__CODE__
}

test();

say "successful /g/" if 'g' =~ /g/;
test();

say "successful /x/" if 'x' =~ /x/;
test();

my $empty='';
say "successful //" if ' ' =~ /$empty/;
test();
[download]

--- run 1
0<>
1<>
2<>
successful /g/
--- run 2
successful /x/
--- run 3
1<x>
2<x>
--- run 4
1<x>
2<x>
[download]

so you are looking for a way to reproduce run 1?

I wasn't able to reset the regex engine here

update

tybalt's suggestions seem to work well:

...
for my $tybalt (qw/ () | (?:) g{0}/) {
    'g' =~ /g/;
    say "successful /$tybalt/" if ' ' =~ /$tybalt/;
    test();
}
[download]

successful /()/
--- run 5
0<>
1<>
2<>
successful /|/
--- run 6
0<>
1<>
2<>
successful /(?:)/
--- run 7
0<>
1<>
2<>
successful /g{0}/
--- run 8
0<>
1<>
2<>
[download]