my %pairs = map { chomp; map @$_ == 2 ? @$_ : (), [split /\t/, $_, 2] } <FH>;

##</code><code>##

my %pairs = map { chomp; map @$_ == 2 ? @$_ : 
                (@$_ == 1 ? $_->[0] => undef : ()),
                [split /\t/, $_, 2] } <FH>;

##</code><code>##

my %pairs = map { chomp; map @$_ == 2 ? @$_ : 
                (@$_ == 1 ? @$_, undef : ()),
                [split /\t/, $_, 2] } <FH>;