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();

##</code><code>##

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

##</code><code>##

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

##</code><code>##

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<>