Splitting into an array simplifies this problem:

#!/usr/bin/perl -w

use strict;

use vars qw($str @s @f);

$str = 'I am a brown icecube';
@s = split('',$str);
@f = map {$_ eq ' ' ? ' ' : '*'} @s;

while (<>)
{
  my $guess=substr($_,0,1);
  foreach my $i (0..$#s)
  {
    if (lc $s[$i] eq lc $guess) { $f[$i] = $s[$i] }
  }
  print @f,"\n";
}
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