Many thanks for all the interest and help here. All the replies were useful.

I really liked these constructs:

s/\".*\"//
my @F = split(/\s+/);
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and

my @F = /(\" .*? \" | \S+)/gx
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But I found the quickest solution was this:

while (<>){

$front = substr( $_, 0, index($_, '"' )-1, "");
$back  = substr( $_, rindex( $_, '"' )+2);
$user_agent = substr ($_, 1, rindex( $_, '"' ));

    $front=~/^([^#\s]+\s\S+)\s(\S+)\s(\S+)\s(\S+)\s(\S+)\s(\S+)\s(\S+)
+\s(\S+)\s(\S+)\s(\S+)\s(\S+)\s(\S+)\s(\S+)\s(\S+)\s(\S+)$/;

        print <<END_FORMAT;

        date time         = $1
        time taken         = $2
        c-ip             = $3
        sc-status         = $4
        ns-action         = $5
        sc-bytes         = $6
        cs-bytes         = $7
        cs-method         = $8
        cs-uri-scheme         = $9
        cs-host         = $10
        cs-uri-stem         = $11
        cs-username         = $12
        s-hierarchy         = $13
        s-supplier-name     = $14
        cs(Content-Type)    = $15
        cs(User-Agent)         = $user_agent
END_FORMAT

        $back=~/^(\S+)\s(\S+)\s(\S+)\s(\S+)\s(\S+)\s*$/;
        print <<END_FORMAT2
        sc-filter-result     = $1
        sc-filter-category     = $2
        x-virus-id        = $3
        s-ip            = $4
            s-sitename        = $5
END_FORMAT2
}
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This processed the following gzip'd log:

bash-2.05b$ ls -l SG*
-rwxr-xr-x    1 js       js       106236830 Mar  7 17:02 SG_CSGL02_mai
+n_470302220000.log.gz
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in 1 minute 32 sec

bash-2.05b$ time gzip -dc SG* | ./test.pl >/dev/null 
6.96user 0.63system 1:32.95elapsed 8%CPU (0avgtext+0avgdata 0maxreside
+nt)k
0inputs+0outputs (94major+33minor)pagefaults 0swaps
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on a 2.6Ghz AMD processor (500MB).