I think the problem might be, the whole list is probably "slurped" into one element of the array only.

Reads file $filename into a scalar. By default it returns this scalar.
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One way is to split:

my @allwords = split /\n/, read_file($ref);
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Once you have your list of dictionary words read from its file and all cleaned up (whitespace, newlines, etc. fixed up), the next step is to realize that very fast lookup of this sort can be had from a hash:

c:\@Work\Perl>perl -wMstrict -le
"use Data::Dump;
 ;;
 my @cleanwords = qw(hut Hat foo HIC);
 my @allwords   = qw(hit Het HAT HiC hAc hoc);
 ;;
 my %dict =
   map { $_ => 1 }
   map canonicalize($_),
   @allwords
   ;
 dd \%dict;
 ;;
 for my $word (@cleanwords) {
   my $common = canonicalize($word);
   printf qq{word '$word' %sin dictionary \n},
     exists $dict{$common} ? '' : 'NOT ';
   }
 ;;
 sub canonicalize { return lc $_[0]; }
"
{ hac => 1, hat => 1, het => 1, hic => 1, hit => 1, hoc => 1 }

word 'hut' NOT in dictionary
word 'Hat' in dictionary
word 'foo' NOT in dictionary
word 'HIC' in dictionary
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Update: Added a few more words to both word lists in example code.

See the Basic debugging checklist: use Data::Dumper or Data::Dump to see what your data structures really look like. I'm guessing you will indeed find that your array contains only one element. The reason seems to be in the File::Slurp::Tiny docs: the documentation for read_file says: "Reads file $filename into a scalar. By default it returns this scalar.", implying that the entire file is read and returned as one scalar. You should probably try using the read_lines function instead. You should probably also turn on the chomp option.

Thanks Anonymous Monk pointing to Data::Dumper. I checked both arrays

The result show clearly, that the structure of the elements is different. An example:

print Dumper @allwords;

...

$VAR9 = 'hat

';

...

print Dumper @cleanwords;

...

$VAR6 = 'hat';

...

I selected the word which should match. In @allwords the single quotation mark appears in a new line. Maybe it's caused by newline? and I should chomp the elements?

I chomped it like this:

chomp (@allwords);

Result:

';AR9 = 'hat

The single quotation mark of the predecessing word appears in the sam line.

Please mark updates to your nodes, see here for why.

I chomped it like this: ... ';AR9 = 'hat ... The single quotation mark of the predecessing word appears in the sam line.

I'd guess the file is terminated by CRLF and your input record separator $/ is a plain LF. One approach to fix this is to set $/ = "\r\n"; before reading the file and the chomp. If you were using a regular open, you could use the :crlf I/O layer: open my $handle, '<:crlf', 'filename.txt' or die $!; (see PerlIO).

Maybe it's caused by newline? and I should chomp the elements?

Yes and yes! :-)

As mentioned below, try setting $Data::Dumper::Useqq=1; before calling Dumper.

You are right.I tried

read_lines

before. But it didn't work either. Thanks for your hint.

P.S.: A step forward: I tried IO::ALL and scalar @allwords reports 10 elements ... But still no matches found.

Perhaps you need to chomp @allwords?

Again, Data::Dumper (especially its Useqq option) or Data::Dump are useful to see the actual strings.

BTW, be careful with IO::All, it is easy to create ETOOMUCHMAGIC errors with it...

$Data::Dumper::Useqq = 1;

This is really amazing!

@allwords

$VAR9 = "hat\r";

@cleanwords

$VAR6 = "hat";

How can I get rid of the \r? I vaguely remember I had this problem before...

Probably this:

s/\r//g;
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(if the variable if contained in $_), or, otherwise, something like this:

$var =~ s/\r//g;
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Je suis Charlie.