push @allPairs, grep { defined } ( $fnum, $snum );

# and at another time .. 

# find half-pairs, which I think is impossible

my @halves = grep { scalar @$_ == 1 && ( $_->[0] == $fnum || $_->[0] == $snum ) } @allpairs;

# find true pairs with a leading fnum

my @matches = grep ( scalar @$_ == 2 && $_->[0] == $fnum } @allpairs

# or a trailing fnum

my @matches = grep ( scalar @$_ == 2 && $_->[1] == $fnum } @allpairs