Sort::Key::Natural will do the trick assuming you can use CPAN (see Yes, even you can use CPAN for tips on how), but there are a couple concepts here that are probably worth learning in any case:

sort lets you define a custom sort routine, like this:

use strict;
use warnings;

my @aChapters=("ch1","ch11","ch2","ch5","ch55","ch16");
my @aSorted = sort {
  my ($sA, $iA) = ($a =~ /(^[^\d]+)(\d+)/);
  my ($sB, $iB) = ($b =~ /(^[^\d]+)(\d+)/);
  my $x = $sA cmp $sB;
  ($x = $iA <=> $iB) unless $x;
  $x;
} @aChapters;

local $"="\n";
print "@aSorted";
[download]

$a and $b are special variables that represent the two members of the list you want to sort. The main trick here is to (i) split your name into two parts: an alpha part and a numeric part (ii) compare the alpha part using the alpha comparison operator (cmp) and the numeric part using the numeric comparison operator (<=>). See perlop for more information.

Best, beth