You ask for no special module, but Term::ReadKey would be much easier. That said:

use POSIX;
use strict;
use warnings;
my $FD = fileno(STDIN);
my $TERM = POSIX::Termios->new();
$TERM->getattr($FD);
my $OTERM = $TERM->getlflag();
my $ECHO = ECHO|ECHOK|ICANON;
my $NOECHO = $OTERM&~$ECHO;
select STDOUT;$| = 1;
sub cbreak {
    $TERM->setlflag($NOECHO);
    $TERM->setcc(VTIME, 1);
    $TERM->setattr($FD, TCSANOW);
}

sub cooked {
    $TERM->setlflag($OTERM);
    $TERM->setcc(VTIME, 0);
    $TERM->setattr($FD, TCSANOW);
}

my $key = '';
my $answer = '';
cbreak();

print "How much is A? ";
while(1) {
    sysread(STDIN, $key, 1);
    last if $key eq "\n";
    syswrite(STDOUT,$key);
    $answer .= $key
}

syswrite(STDOUT," A:$answer\n");
cooked();

END { cooked() }
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