You are suffering from buffering. Here are two fixes:

use IO::Handle qw( );
STDOUT->autoflush(1);

do {
    sleep(5);
    print "Hello...";
} while(1);
[download]

use IO::Handle qw( );

do {
    STDOUT->flush();
    sleep(5);
    print "Hello...";
} while(1);
[download]