Your line my @data = $_[0]..$_[scalar(@_)-2]; isn't doing what you think it is, it's passing @data a list of values using the first number in the original @data as the start of a range, and the last number as the end of a range, thereby you end up with a list of numbers from 1 to 9, probably the simplest fix would be to reverse the order you are passing the arguments in and grab them in a much simpler fashion. ie:

#!c:\perl\bin

use strict;
use warnings;

my @data = (1,-5,0,-8,2,0,4,7,3,6,9);

@data = sortData("Integer",@data);

sub sortData
{
  my ($dataType, @data) =@_;

  if($dataType  eq 'Integer' || $dataType eq 'Float')
  {
    sort {$a <=> $b} @data;
  }
  else
  {
    sort @data;
  }
}
[download]

This code will work as you expect it to, though I'm sure other monks will follow with solutions involving pops and the like so you can continue to call the sub in the same fashion...