Your 2 numbers might have different internal binary representations. See example below:

my $x='-3809482.4215422305'+0;
my $y='-3809482.4215422301'+0;

my $diff = abs( $x - $y );
print $x,"\n",$y,"\n",$diff,"\n";
[download]

Output:

-3809482.42154223
-3809482.42154223
4.65661287307739e-10
[download]

(perl 5.6.1 on linux)