It's floating point error. See for example What Every Computer Scientist Should Know About Floating-Point Arithmetic.

$ perl -wMstrict -le 'my $a=5.10*100; print "plain: ",$a, "\nint: ",
  int($a); printf "%%d: %d\n%%f: %.20f\n",$a,$a;'
plain: 510
int: 509
%d: 509
%f: 509.99999999999994315658
[download]

One simple built-in way to round numbers in Perl is with sprintf, i.e. something like sprintf("%.0f",$a). For exact math in Perl, some examples are Math::BigInt, Math::BigFloat and Math::BigRat.