use strict;

my $info     = "I am a brown icecube";
(my $unknown = $info) =~ tr[a-zA-Z][*];

until($info eq $unknown) {
  print "guess a char: ";

  chomp(my $input = <STDIN>);
  my $c = substr $input => 0, 1;

  next
    if $c !~ /[a-z]/i
       or index($info => $c) == -1;

  substr($unknown => pos($info) - 1, 1) = $c
    while $info =~ /$c/g;

  print $unknown, $/;
}