Term::InKey is a good solution to the OP question, especially if he really does want to be able to modify characters before echoing, and he's just giving passwords as an example of this. One common use is uppercasing a sign-on or other field to indicate that it's case-insensitive.

When doing this, remember to unbuffer STDOUT, as in:

use warnings;
use strict;
use Term::InKey;
use IO::Handle;
    STDOUT->autoflush(1);

my $x='';

print "receiving chars\n";

while ($x ne 'x') {
   $x = ReadKey();
   print uc($x);
}
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Hope this helps.