my %count;
while (...) {
 my($field1, $field2) = split(...);
 # assume that we already has the key $field1 in %count
 # so the value would be $count{$field1}.
 #
 # assume it to be array reference, pretend you did:
 # $count{$field1} = []
 #
 # the push() function requires real array so you need
 # to dereference it first, like this @{$count{$field1}},
 # then push the new value
 #
 push @{$count{$field1}}, $field2;
}