Disproven:

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

use Test::More tests => 1;

my $v_amount    = -683.12;
my $plbamt      = -683.12;
my $finalamount = $v_amount - ($plbamt); # Why have brackets here, OP?
is ($finalamount, 0, 'Nothing to see here');

[download]
can anyone tell me why this is?

Yes, it is because at least one of $v_amount or $plbamt does not have the value which you think it has. See the links the other monks have given you regarding imprecision of floating point numbers in computing.

In reply to Re: subtraction issue by hippo
in thread subtraction issue by Anonymous Monk

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