my $input = '(?{ print("Hello World!\\n") })';

print(qq{Without "use re 'eval';":\n});
eval {
   '' =~ /$input/;
};
warn($@) if $@;

print("\n");

print(qq{With "use re 'eval';":\n});
eval {
   use re 'eval';
   '' =~ /$input/;
};
warn("Died: $@") if $@;

##</code><code>##

Without "use re 'eval';":
Died: Eval-group not allowed at runtime, use re 'eval' in regex m/(?{ print("Hello World!\n") })/.

With "use re 'eval';":
Hello World!