#!/usr/bin/perl
do { print "CALC> ";
     printf "%f\n", eval( Filter(<>) );
   } until 0;

sub Filter {
    my $exp = shift;
    ( $exp =~ /^\s*exit/ ) and return $exp;
    ( $exp =~ /^\s*(\w+)/ )
        and return 'print STDERR "$1 denied\n"';
    return $exp;
}