How about something like this?

$chars will just contain a list of those you want to expose, everything else is converted to an asterisk or what ever you want. You may want to keep an original as I did with $info. Then use something like $known to expose the letters as desired. Notice, I included a space in the reveal chars to match your example.

I hope this helps :-)

#!/usr/bin/perl -w

use strict;

my $info   = "I am a brown icecube";
my $chars  = 'a ';

my $known  = $info;
$known =~ s/[^$chars]/\*/g;

print "$info\n";
print "$known\n";
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