If you want to be able to change any number in the file name, you would probably like to have it split into chunks. So far my idea; I realize that shenme has already written a piece of code utilizing this, but TIMTOWDI, and so I decided to write another piece of code:

#!perl -w

use strict;

for my $name (<DATA>) {
   chomp $name;
   my %parts;
   $name =~ s/(.*)(\.[^.]*)/$1/;
   $parts{"suffix"} = $2 or die "Not a valid file name: $name";
   $parts{"prefix"} = [];
   $parts{"number"} = [];
   while ($name =~ s/^(\D*)(\d+)//) {
      push @{$parts{"prefix"}}, $1;
      push @{$parts{"number"}}, $2;
   }
   $name and die "Invalid file name format. Rest '$name' remained";
   print "Filename splits as follows: [" 
       . join("][", 
            map { "(" . $parts{"prefix"}->[$_] . ")(" 
                      . $parts{"number"}->[$_] . ")" } 0..@{$parts{"nu
+mber"}}-1)
       . "]<"
       . $parts{"suffix"} . ">\n";
}

__DATA__
01-html02.html
01-htm23-43.htm
01-file-01.html
[download]

The program first extracts the file suffix (file ending after the dot, I hope that I didn't misunderstand you here) and then loops through the file name, taking (possibly) a prefix and (definitely) a number from it and storing it in anonymous arrays in $parts{"prefix"} and $parts{"number"}.
If you want to increment the $ith number now, you would just have to write

++$parts{"number"}->[$i];
$filename = join('',
               map { $parts{"prefix"}->[$_]                    
                   . $parts{"number"}->[$_] }                  
                               0..@{$parts{"number"}}-1)        
            . $parts{"suffix"};
[download]

Hope that helped.