Alright, perhaps try something along these lines:

#! /usr/bin/perl
use strict;
use warnings;

my $file = shift @ARGV;
my @times = map { /(\d+)_seconds/; $1 }
            grep { /$file/ } <DATA>;
my $totaltime; $totaltime += $_ for @times;
my $avgtime = $totaltime / @times;
print "Average time: $avgtime\n\n";

__DATA__
[24/Mar/2004:12:26:52 /manual/misc/perf-tuning.html 0_seconds
[24/Mar/2004:12:27:33 /manual/mod/mod_status.html 0_seconds
[24/Mar/2004:12:27:39 /manual/mod/module-dict.html 0_seconds
[24/Mar/2004:12:27:46 /manual/misc/rewriteguide.html 0_seconds
[24/Mar/2004:12:27:53 /manual/mod/mod_rewrite.html 5_seconds
[24/Mar/2004:12:27:53 /manual/images/mod_rewrite_fig1.gif 0_seconds
[24/Mar/2004:12:27:53 /manual/images/mod_rewrite_fig2.gif 0_seconds
[24/Mar/2004:12:28:05 /manual/new_features_1_3.html 0_seconds
[24/Mar/2004:12:29:53 /manual/mod/mod_rewrite.html 6_seconds
[24/Mar/2004:12:29:54 /manual/mod/mod_rewrite.html 7_seconds
[24/Mar/2004:12:29:55 /manual/mod/mod_rewrite.html 8_seconds
[24/Mar/2004:12:29:56 /manual/mod/mod_rewrite.html 9_seconds
[download]

I still think you should try the format manipulation in Perl, though; it's easy to do, and you'll only have one script to maintain.