use strict; 
use warnings; 
open my $B, '<', 'file_B.txt' or die "Cannot open file_B: $!"; 
my @functions; 
my %toc; 
my $i =0; 
while (my $line = <$B>) { 
    chomp $line; 
    $line =~ s/(\d+)\s//; 
    $functions[$i] = $line; 
    $toc{$1} = $i++; 
} 
close $B; 
open my $A, '<', 'file_A.txt' or die "Cannot open file_A: $!"; 
open my $C, '>', 'file_C.txt' or die "Cannot open file_C; $!"; 
my $prev_id = 'foo'; 
while (my $line = <$A>){ 
    my ($id,$code) = split /\s+/, $line; 
    if ($id ne $prev_id){ 
        print {$C} "\n" if $prev_id ne 'foo'; 
        print {$C} "$id "; 
        $prev_id = $id; 
    } 
    else { 
        print {$C} "\t"; 
    } 
    print {$C} $functions[$toc{$code}]; 
} 
print {$C} "\n"; 
close $C; 
close $A;

K01369 <tab> PATH:ko00350; Metabolism; Enzyme Families; Peptidases <tab> BR:ko00199; Cellular Processes; Transport and Catabolism; Lysosome <tab> PATH:ko00350; Organismal Systems; Immune System; Antigen processing and presentation

        print {$C} "$id\t";

        print {$C} ",\t";