Although the issue at hand might be to correct the above example code, forgive me if I mention the obvious - i.e. that for evaluating formulas read in as data, the eval function does seem rather suitable. Because it is functionally overkill, a filter function is also supplied in this quickie example:

#!/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;
}
[download]

example session:

CALC> 0.7*0.9

0.630000

CALC> 0.6-0.8

-0.200000

CALC> mkdir freddy;

mkdir denied

CALC> exit

$

-S