Basically what it does is it loops randomly through A-Za-z and a space until it finds the correct character for that spot and then moves on to the next character. I added a small delay too so one can see it shuffling through the characters instead of the boring, ordinary way of instantaneous appearance of the whole string. ;)

use Time::HiRes 'usleep'; # to get usleep

++$|; # same as $| = 1, i.e. enabling autoflush

$s='rekcah lreP rehtona tsuJ'; # "Just another Perl hacker", reversed

@v=(65..90,97..122,32); # ASCII values for A-Z, a-z and a whitespace

do{
  do{
    # setting $_ to a random letter from @v, printing it
    print $_ = sprintf '%c',$v[rand@v];

    # sleeping for a while
    usleep 2000;

    # printing \b, a backspace
    print"\b"

  #..while $_ is not the last letter of $s
  } while $_ ne substr $s,-1;

  # shortening $s by one, i.e. moving on to the next letter
  chop $s;

  # printing $_, i.e. the correct letter for that space
  print

#..while there's still something to print in $s
} while $s
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