Actually, I think it would be better to just get the absolute value:

$balance_due = abs($balance_due);
[download]

This protects you in the event that $balance_due is a positive number (which shouldn't happen according to the SQL, but I've always beleived that a good programmer is someone who looks both ways on a one way street). It's also close enough in speed to negation that random noise is a factor in execution time. Code:

#!/usr/bin/perl
use strict;
use warnings;
use Benchmark qw(cmpthese);

my $data  = -12345;
my $count = pop || 500000;

cmpthese($count, {
    negate   => sub { my $i = -$data }, 
    absolute => sub { my $i = abs($data) }, 
    regex    => sub { my ($i) = $data =~ s/-// }, 
});
[download]

Results:

Benchmark: timing 500000 iterations of absolute, negate, regex...
  absolute:  1 wallclock secs ( 1.24 usr +  0.00 sys =  1.24 CPU) @ 40
+4858.30/s (n=500000)
    negate:  1 wallclock secs ( 1.15 usr +  0.00 sys =  1.15 CPU) @ 43
+3275.56/s (n=500000)
     regex:  3 wallclock secs ( 3.16 usr +  0.00 sys =  3.16 CPU) @ 15
+8328.06/s (n=500000)
             Rate    regex absolute   negate
regex    158328/s       --     -61%     -63%
absolute 404858/s     156%       --      -7%
negate   433276/s     174%       7%       --
[download]

----
I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
-- Schemer

Note: All code is untested, unless otherwise stated