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 ++;
            }
}
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For more information, you may want to check out Coping with Scoping by our own Dominus.

HTH