What I use on Unix:

$ cat renum
#! /usr/bin/perl -w
use strict;

my $len = 0;
if ($ARGV[0] =~ /^\d+$/)
{
    $len = shift;
}
elsif ($ARGV[$#ARGV] =~ /^\d+$/)
{
    $len = pop;
}
else
{
    foreach my $f (@ARGV)
    {
        $f =~ /(\d+)(\D*)$/;
        $len = length $1 if length $1 > $len;
    }
}

unless ($len)
{
    print "No length given/discovered\n";
    exit 1;
}

foreach my $f (@ARGV)
{
    (my $newf = $f) =~ s/(\d+)(\D*)$/sprintf "%0${len}d%s", $1, $2/e;
    if ($newf ne $f)
    {
        print "$f -> $newf\n";
        rename $f, $newf;
    }
}
[download]

However, if you want this to work cleanly on Windows, too, I'd just add a line near the top like this: @ARGV = map { glob $_ } @ARGV) and then you could run this as perl renum e:/perl/comics/*.png - it would figure out that the largest number is 3 digits long, and renam all of them to be 3 digits long. Slightly more generic.