It's easy, you should turn on autoflush on STDOUT

use strict;
use warnings;
use IO::Handle;

autoflush STDOUT 1;
my $str = "Hello!";
chomp($str);
foreach ( split //, $str ) {
    print $_, " ";
    sleep 1;
}
print "\n";
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Update: or use $|=1 as proposed by gwadej, in case of SDTOUT that perhaps better solution.