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

while (<DATA>) {
    chomp;
    my @fields = split /\s+/; # Change to /\t/ for production?


    # You can walk the array like this:
    print "\t$fields[0]\n";
    for my $i ( 1 .. $#fields ) {
        print "$fields[$i-1]\t$fields[$i]\n";
    }
    # Or like this:
#    my $last_field = '';
#    for my $field (@fields) {
#        print "$last_field\t$field\n";
#        $last_field = $field;
#    }


    print "\n";
}

__DATA__
A   B   C   D   E
Foo Bar Baz
Senior  Junior  TheThird

##</code><code>##

    A
A   B
B   C
C   D
D   E

    Foo
Foo Bar
Bar Baz

        Senior
Senior  Junior
Junior  TheThird