That snippet uses readdir in list context. You'd want

my $dn = '.';
my @f;
opendir(my $dh, $dn) or die;
while ( @f < 5 && defined( my $filename = readdir($dh) ) ){
   next if $filename=~/^\.\.?\z/;
   push @f, $filename;
}
[download]

Other changes:

• I renamed $abs_d cause it didn't contain an absolute path.
• I used @f<5 rather than $#f<4 cause there's no need to hide the actual number of interest.
• I used a lexical instead of a global variable for the handle. Needless use of globals is bad.
• Your check for "." and ".." was flawed. It could skip legitimate file names.

Note that you'll quite likely get the same files in the same order every time. But that's how it's going to be unless you read the whole dir.