for my $idx (1..4) {
   ...
   fork
   ...
   say $dirnames[$idx - 1];
   ...
}

##</code><code>##

for my $idx (1..@dirnames) {
   ...
   fork
   ...
   say $dirnames[$idx - 1];
   ...
}

##</code><code>##

for my $idx (0..$#dirnames) {
   ...
   fork
   ...
   say $dirnames[$idx];
   ...
}

##</code><code>##

for my $dirname (@dirnames) {
   ...
   fork
   ...
   say $dirname;
   ...
}