you cant, without modules. Infact what can terminate your input if not a newline ( or a CTRL-Z on empty line )?

But with Term::ReadKey you can read a char at time:

use strict;
use warnings;

use Term::ReadKey;
$|++;  #you must autoflush buffer or it print only at newlines
ReadMode('cbreak');
print "How much is A:";
my $input;
my $char;
while ( $char = ReadKey(0) and $char =~/^\d$/ ){
    $input.=$char;
    print $char;
}
print " A=$input\n";
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In the above code any non digit entered by the user will stop the while loop and you can print on the same line.

L*