#!/usr/bin/perl
use warnings;
use strict;
use Fcntl ':seek';


open DATASET_A, '<', 'File_A' or die "Cannot open 'File_A' $!";
open DATASET_B, '<', 'File_B' or die "Cannot open 'File_B' $!";


my %dataB;

my $tell = tell DATASET_B;
while ( <DATASET_B> ) {
    if ( /^(\S+)/ ) {
        push @{ $dataB{ $1 } }, $tell + length $1;
        }
    $tell = tell DATASET_B;
    }


while ( <DATASET_A> ) {
    chomp;
    my ( $key ) = /^(\S+)/;
    if ( exists $dataB{ $key } ) {
        for my $seek ( @{ $dataB{ $key } } ) {
            seek DATASET_B, $seek, SEEK_SET;
            print $_, scalar <DATASET_B>;
            }
        }
    }


__END__