Please try to provide good sample code for people new to Perl. $a and $b are special variables and are poor choices for variable names in any case. Always use strictures (use strict; use warnings;). Use the three parameter version of open and use lexical file handles. Declare variables in the smallest scope that makes sense. Try to write a self contained example that can be run. Consider:

use strict;
use warnings;

my $fileName = 'data.txt';

# Create the sample data file
open my $outFile, '>', $fileName or die "Failed to create $fileName: $
+!";
print $outFile <<SAMPLEDATA;
Db10g029860.2 7
Db10g029860.2 1
Db10g029860.2 2
Db10g029860.2 1
Db10g029860.2 4
Db10g029860.2 2
Db10g029860.2 6
Db10g029860.2 11
Db96g938791.0 5
Db96g938791.0 9
Db96g938791.0 1
Db96g938791.0 3
Db96g938791.0 7
Db04g787390.1 6
Db04g787390.1 5
Db04g787390.1 5
Db04g787390.1 12
SAMPLEDATA
close $outFile;

# Process the input file
open my $fileIn, '<', $fileName or die "Unable to open $fileName: $!";

my %counts;

while(<$fileIn>) {
    chomp;
    my ($id, $count) = split /\s+/;

    next if ! $count;
    $counts{$id} += $count;
}

# Print the result
for my $id (sort keys %counts) {
    printf "%-12s: %5d\n", $id, $counts{$id};
}
[download]

Prints:

Db04g787390.1:    28
Db10g029860.2:    34
Db96g938791.0:    25
[download]