How come when I run this script ... it works just fine, but when I try using it in a bigger program it gives me the ":No such file or directory found at lingo4.pl line 508" error?

Because cwd is cwd and and readdir doesn't return absolute paths

If you opendir /a/s/d/f and readdir returns . .. foo.txt, then if you try to open foo.txt, you're trying to open cwd()/foo.txt not /a/s/d/f/foo.txt, so open will fail

you should have used File::Find::Rule

Because your bigger program is unwieldy

subroutine declarations are mixed with the main code -- write a single subroutine called Main, and confine the main logic of your program there -- see template at (tye)Re: Stupid question (and see one discussion of that template at Re^2: RFC: Creating unicursal stars

your code is 11 times as long as it should be -- you know how to make subroutines, so write subroutines, don't copy/paste the same code (same loops) 11 times

You're using short and/or meaningless variable names and putting the meaning in comments -- just use meaningful variable names :)

(sub folder) you already know how to use hashes, so don't code long if/else structures to return an integer, use a hash

(calling folder) dozens of while loops are not a substitute for a giant if/else structure -- which you wouldn't need if sub folder teturned a string instead of an integer

or just have sub folder take care of the printing

and while you're at it "folder" is a terrible name for a "sub" -- call it sub processFolder

If you have 14 minutes, watch String Calculator TDD Kata done in Perl , then rewrite your code in a similar fashion

my %korpusEng;
sitKorpusInHash( \%korpusEng , 'korpusENG.txt' )l
...
[download]

Another example Out of memory and While replacements with excel XLSX application / Re^2: Out of memory and While replacements with excel XLSX application

good luck