my %hash = (
 a => 1,
 b => 2,
 c => 3,
 d => 4
);

sub foo
{
  my $n = ($_[0]) ? $_[0] : keys %hash;
  return (keys %hash)[0 .. ($n - 1)]; # return %hash keys from 0 to $n - 1
}

my $bar = &foo(); # want to return the number of keys of %hash
print "$bar\n";