$ perl -wle '"" =~ /(?{print "Fooled you!"})/'
Fooled you!
$ perl -wle 'use re "eval"; my $re = shift; "" =~ /$re/' '(?{print "Fooled you!"})'
Fooled you!
$ perl -wle 'my $re = shift; "" =~ /$re/' '(?{print "Fooled you!"})'
Eval-group not allowed at runtime, use re 'eval' in regex m/(?{print "Fooled you!"})/ at -e line 1.