my $re;
my $str = '(foo';
eval {$re = qr/$str/};
if ( $@ ) {
    print "Oops $@\n";
}
print "Still running.....$re\n";

$str = '/;print"You hacked me!";/';
eval {$re = qr/$str/};
if ( $@ ) {
    print "Oops $@\n";
}
print "Still running.....$re\n";

$str = '/;print"You hacked me!\n";/';
eval "\$re = qr/$str/";
if ( $@ ) {
    print "Oops $@\n";
}
print "Still running.....$re\n";
__DATA__
Oops Unmatched ( before HERE mark in regex m/( << HERE foo/ at C:\PROGRA~1\PERLBU~1\debug\test.pl line 5.

Still running.....
Still running.....(?-xism:/;print"You hacked me!";/)
*****You hacked me!
Still running.....(?-xism:)