Adding $| = 1; (see Suffering from buffering) and replacing your \n with a space:

#!/usr/bin/perl

use strict;
use warnings;

$| = 1;
my $str = "Hello!";
foreach ( split // , $str ){
  print "$_ "; #note the space
  sleep 1;
}
print "\n";
[download]

Will do what I think you want.

Martin