# Define our buckets for collecting data
my %bucket  = ();
my %USER    = ();
my %VIRUS   = ();
my %SPAM    = ();
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my creates a variable in an empty state. Assigning emptiness to an empty container is redundant.

# These numbers must be valid reason codes...
my @wanted = ( 1, 60 );
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It would be more efficient to use a hash instead of an array there.

for (<$file>)
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A for loop iterates over a list which means that the entire contents of the file have to be stored in a list in memory before it can be iterated over. It is more efficient to use a while loop which only reads one line at a time.

while ( <$file> )
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    chomp;    

    my ( $month, $day, $time, $host,
         $process, $clientIP, $MessageID,
         $timeStart, $timeEnd, $service, @INFO ) = split /\s+/;
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The chomp is superfluous because split /\s+/ removes all whitespace. You are declaring eleven variables but you are only using three. You can use undef as a placeholder:

my ( undef, undef, undef, $host, undef, undef, undef, undef, undef, $s
+ervice, @INFO ) = split;
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        $USER{$host}{$recip}{$action{$service}{$action_}}{$reason{$rea
+son_}}{'count'}++;
            #if ( grep /^$reason_$/, @wanted );
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If you had used a hash for wanted then you would not need to use grep.

        $USER{$host}{$recip}{$action{$service}{$action_}}{$reason{$rea
+son_}}{'count'}++;
            #if exists $wanted{ $reason_ };
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