Instead of dereferencing the entire multilevel structure each time, if you create an array of references to the scalars you are incrementing, then you can iterate over that array and increment the values in the nested hash in 1/6th the time:

#! perl -slw
use strict;
use Time::HiRes qw[ time ];

my %DB_STATS;
$DB_STATS{ COMB }{ $_ } = { Rit=>0, Usc=>0 } for 1 .. 18e5;

my $start = time;
++$DB_STATS{ COMB }{ $_ }{Rit} for 1 .. 18e5;
printf "Full deref took %.3f seconds\n", time() - $start;

my @Rits;
$Rits[ $_ ] = \$DB_STATS{ COMB }{ $_ }{Rit} for 1 .. 18e5;

$start = time;
++$$_ for @Rits;
printf "AoR via alias took %.3f seconds\n", time() - $start;

print $DB_STATS{COMB}{1}{Rit};

__END__
C:\test>910049
Full deref took 1.511 seconds
AoR via alias took 0.247 seconds
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