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$balance_due = abs($balance_due);

#!/usr/bin/perl
use strict;
...
    absolute => sub { my $i = abs($data) }, 
    regex    => sub { my ($i) = $data =~ s/-// }, 
});

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)
...
regex    158328/s       --     -61%     -63%
absolute 404858/s     156%       --      -7%
negate   433276/s     174%       7%       --