This is indeed a string that contains a qr expression:

   {"name" => 'qr/.*uba$/i'}
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Does removing the quotes solve your problem?

   {"name" => qr/.*uba$/i}
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When you perform string manipulations to form your qr-expression, you might find quotemeta useful.

i figured it out. The issue was that when i created the $re variable, it was adding the slashes. by using just what was inside the slashes of the original string, i managed to get it to work

            if(index($_[0],"qr/") >= 0){ 
                $_[0] =~s/qr[\/](.*?)([^\\])[\/](i?)$/$1$2/s;
                print $fh Dumper($_[0]);
                if($3=='i'){
                    $_[0] = qr/$_[0]/i;
                }else{
                    $_[0] = qr/$_[0]/;
                }
                print $fh Dumper($_[0]);
            }
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Maybe use split

my $string = '/.*uba$/i';
my @f = split '/',$string;
my $qr = ($f[-1] eq 'i') ? qr/$f[1]/i : qr/$f[1]/;

my $cursor = $collection->query( { name=> $qr });
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poj

Not an answer, just a comment. Do I get it right:

• "setter" (in quotes, because I understand it's not really a setter) accepts a compiled regex, one you build with the qr operator;
• "getter" returns not this compiled regex, but, somehow, Perl source code fragment, literal string 'qr/.*uba$/i'. Which you then are trying to parse. Or, where does this 'qr/.*uba$/i' come from, and why is it necessary to manually parse Perl expression?

It doesn't make much sense to me, but I haven't worked with MongoDB, hence not an answer, but a promised comment. The reason you get an undef in OP, is because $re are 2 separate lexical variables, 1 per each own small block. They are declared with my and destroyed at closing curly bracket. What's assigned to $_[0], in next line, is presumably global, completely different $re, and undefined at that time. You don't use strict;, do you? I think this change

my $re;
if($2 eq 'i'){
    $re = qr/$_[0]/i;
}else{
    $re = qr/$_[0]/;
}
$_[0] =$re;
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would give you the result you were hoping for.

Edit: Fixed numeric comparison operator to string comparison eq (thanks, poj).

Then put "(?$flags:" in front of it, and ")" behind it. You can use that string ($re) directly as a regex:

$re = "(?:$flags:$core)";
if($input =~ $re) { ... }
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$re = "(?$flags:$core)";
if($input =~ $re) { ... }
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$qr = qr/$re/;
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$re = "(?:$flags:$core)";

That doesn't work: too many  ':' (colons):

c:\@Work\Perl\monks>perl -wMstrict -le
"my $string = '/.*uba$/i';
 ;;
 my $flag = qr{ [msixpodual] }xms;
 ;;
 my $convertable =
 my ($core, $flags) = $string =~ m{
   \A \s* / (.*?) / ($flag*) \z
   }xms;
 die qq{bad rx: '$string'} unless $convertable;
 ;;
 my $re = qq{(?:$flags:$core)};
 print $re;
 print 'A: match' if 'uba' =~ $re;
"
(?:i:.*uba$)
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Something like this works:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $string = '/.*u\/ba$/i';
 ;;
 my $flag = qr{ [msixpodual] }xms;
 ;;
 my $convertable =
 my ($pattern, $modifiers) = $string =~ m{
   \A \s* / (.*?) / ($flag*) \z
   }xms;
 die qq{bad rx: '$string'} unless $convertable;
 ;;
 my $rx = qr{(?$modifiers)$pattern};
 print $rx;
 print 'A: match' if 'u/Ba' =~ $rx;
 ;;
 my $ry = qr{ \A foo $rx }xms;
 print $ry
 ;;
 print 'B: match' if 'foolubatU/bA' =~ $ry;
 print 'C: match' if 'FoolubatU/bA' =~ $ry;
"
(?^:(?i).*u\/ba$)
A: match
(?^msx: \A foo (?^:(?i).*u\/ba$) )
B: match
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Note that:
• I go directly to  qr// form.
• The modifiers ($flag pattern) implemented vary by Perl version; see perlop.

---

Give a man a fish:  <%-{-{-{-<

Yes, "(?:$flags:$pattern)" is wrong, it should have been "(?$flags:$pattern)". That is: the flags between the leading "(?" and ":" of the standard non-capturing grouping parens, i.e. "(?:$pattern)".

You can always try to print out a qr/foo/i, to see what perl does natively.