use strict;
use warnings;
my %hash;
while (<DATA>) {
    my @elements = split;
    $hash{join ' => ', @elements[0,1]}++;
    $hash{$elements[0]}++;
}
foreach my $key (sort keys %hash) {
    printf "%-6s  =>  %d\n", $key, $hash{$key};
}
__DATA__
A B n1
A B n2
A C n1
D E n2
D E n4
D F n1