Some indentation goes a long way. Excessive comments hurt more than they help. The dot in your regex is not matched literally, it's a wildcard for "(nearly) any character". You're slurping the file, which could easily lead to a lot of memory consumption on busy sites with large logfiles. Testing $#ARGV including equality to zero means you have to pass at least two parameters, since with a single element the result will be 0 and trigger your condition.

#!/usr/bin/perl -w
use strict;

@ARGV = qw(/var/log/apache/access_log) unless @ARGV;

my %source;

while(<>) {
    next unless /cmd\.exe/;
    my ($ip) = /^(\S+)/;
    $source{$ip}++;
}

printf "%-6s %10s\n", $source{$_}, $_ for keys %source;
printf "%15s %10s\n", "Total:", scalar keys %source;
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Untested.

Makeshifts last the longest.