Note that you need to make a close() after run your code, or you won't have the output flushed! Or you can just use flush in the creation.

Soo, your code should be:

use Safe::World;

my ($stdout, $stderr);

my $world = Safe::World->new(
    stdout => \$stdout,
    stderr => \$stderr,
);

$world->eval('print 12 ** 2, "\n";');

$world->close ;

print $stdout ;
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Or using flush:

use Safe::World;

my ($stdout, $stderr);

my $world = Safe::World->new(
    stdout => \$stdout,
    stderr => \$stderr,
    flush  => 1 ,
);

$world->eval('print 12 ** 2, "\n";');

print $stdout ;
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