Yes, IEEE floating point numbers aren't exact representations. Use Math::BigFloat:

pp2@nereida:~/src/perl/testing$ cat -n addlong.pl
     1  use warnings;
     2  use strict;
     3  use Math::BigFloat ':constant';
     4
     5  my $x=100.0;
     6  my $y=-100.01;
     7  print (($x+$y)."\n");
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And now:

pp2@nereida:~/src/perl/testing$ perl addlong.pl
-0.01
[download]

Best

Casiano