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;
}

##</code><code>##

my %count;
while (...) {
  my($field1, $field2) = split(...);
  if (exists $count{$field1}) {
    # force the value to be array ref if it's
    # not done so
    $count{$field1} = [$count{$field1}]
      unless ref $count{$field1};
    push @{$count{$field1}}, $field2;
  }
  else {
    $count{$field1} = $field2; # normal assignment
  }
}

##</code><code>##

print Dumper(\%count);