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();