#!/usr/bin/env perl
use 5.010; use strict; use warnings;
use Data::Printer;

my @l; # array of hashes, to hold the keys and values for each line
my %a; # hash to keep track of all keys
my %c; # hash to track whether a key changes value
       #   if a key is still in the hash when the loop finishes,
       #      that means it had the same value throughout and should
       #      be ignored

while(<DATA>){
    chomp;
    my %h = map { split /=/,$_,2 } split '&'; # split string into keys/values
    push @l, \%h;
    if( $. == 1 ){                   # on the first line, load into %c
        %c = %h;
    } else {                         # on other lines, check values
        for my $k (keys %h){
            delete $c{$k} unless exists $c{$k} and $c{$k} eq $h{$k}; # remove if different
            $a{$k}=1;   # keep track of key existing
        }
    }
}
# remove consistent keys from output
delete $a{$_} for keys %c;

for my $l (@l){
    for my $k (sort keys %a){
        printf "%-8s", $l->{$k} ? "$k=$l->{$k}" : ' ';
    }
    say '';
}

# I've mixed up the data a bit to reflect my open requirements.
# a & e should still be dropped from the output.
__DATA__
a=1&b=1&f=3&c=1&d=2&e=
a=1&b=2&c=3&e=&f=4
b=2&a=1&c=5&d=1&e=&f=5