This is a solution that works in ActiveState Perl v5.8.8

#!/usr/bin/perl
use warnings;
use strict;

my $nz = -0.000000000001;

$nz = sprintf("%.5f",$nz);

print $nz, " ", zerofy($nz), "\n";

sub zerofy {
   my $num = shift;
   if ( $num == 0.0 ) {
       $num = 0.0;
   }
   return $num;
}
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The output is:

C:\Code>perl zerofy.pl
-0.00000 0
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