c:\@Work\Perl\monks>perl -wMstrict -le
"my $string = '/.*u\/ba$/i';
 ;;
 my $rx = eval 'qr' . $string;
 print $rx;
 print 'A: match' if 'u/Ba' =~ $rx;
 ;;
 my $ry = qr{ \A foo $rx }xms;
 print $ry;
 print 'B: match' if 'foolubatU/bA' =~ $ry;
"
(?i-xsm:.*u/ba$)
A: match
(?msx-i: \A foo (?i-xsm:.*u/ba$) )
B: match
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Update: Of course, given that you're dealing with user-supplied data and considering the danger of an injection attack, maybe one should think twice about an eval-based approach.

In addition to what AnomalousMonk said. Plain regular expressions can be constructed to be DoS attacks; even unintentionally. The handling varies from Perl version to version and I am pretty sure regular expressions were impossible to interrupt with a timeout historically so the "attack" was impossible to guard against. I have no idea if this is still the case in newer versions or when it changed if it did.