You can't my a single hash element. Declare the entire hash once, outside the loop. Move the declaration for $smtp_inc_tot outside the loop as well. Something like this:

my (%smtp_inc_src_cnt, %smtp_inc_dst_cnt, $stmp_inc_tot);
if ($if_name eq $outside_if && $if_dir eq 'inbound') {
            if (my $service eq 'mail') {
                           $smtp_inc_src_cnt{$src} ++;
                           $smtp_inc_dst_cnt{$dst} ++;
                           $smtp_inc_tot ++;
            }
}
[download]

HTH

A few comments on your code; firstly, the line if (my $service eq 'mail') has me confused. You're making a new variable named $service and then immediatly checking if it's equal to 'mail'. Being a new variable, however, its value is undef still. Perhaps you got overzealous with the mys?

Lastly, we come to the lines which are actually raizing your error. $smtp_inc_src_cnt{$src} is just one element of a hash -- which, if you think about it, would be rather hard to make a new variable of (what happens if you've already defined the rest of the hash elsewhere?) Perhaps you meant:

my %smtp_inc_src_cnt = (src => 1);
my %smtp_inc_dst_cnt = (dst => 1);
my $smtp_inc_tot = 1;
[download]

But even that doesn't do what you want -- the hashes and scalar would be only defined within your innermost if statement. So both of them should probably be moved to outside you loop, like so:

my (%smtp_inc_src_cnt, %smtp_inc_dst_cnt,$smtp_inc_tot);
if ($if_name eq $outside_if && $if_dir eq 'inbound') {
            if ($service eq 'mail') {
                           $smtp_inc_src_cnt{$src}++;
                           $smtp_inc_dst_cnt{$dst}++;
                           $smtp_inc_tot++;
            }
}
[download]

Update: *sigh* -- never node just after waking up. Messing up precedence is something I hardly ever do. Grump. Kudos to chipmunk for poking me.

perl -pe '"I lo*`+$^X$\"$]!$/"=~m%(.*)%s;$_=$1;y^+*`^ ve^#$&V"+@( NO CARRIER'