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');
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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.