#! perl -sw
use strict;

my $pos = tell DATA;
my %tally; ++$tally{ $_ } for map split( '&' ), <DATA>;
my $lines = $.;
seek DATA, $pos, 0;

print join '&', grep{ $tally{ $_ } != $lines } split '&' while <DATA>

__DATA__
a=1&b=1&c=1&d=2&e=&f=3
a=1&b=2&c=3&d=2&e=&f=4
a=1&b=2&c=5&d=1&e=&f=5