See how tac thru a piped open works speedwise for you.

#!/usr/bin/perl
use warnings;
use strict;

my $file = shift || $0;

my $pid = open(FH,"tac $file |") or die "$!\n";

my $count = 0;
while (<FH>){
 #do your line processing here
 print $count++ .' '. $_; 
}
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