If it is not tons of lines...
It would also be good if it runs standalone without requiring any extra inputs, files etc.

I hope you dont considered those code as ton of lines codes. and the input file is just the list of directory which should be scanned

I have tried to clean it up a bit but no guarantee as I cannot run it...

• I have changed the loop over the files to eliminate $number.
• I also eliminated $num as $. already contains the line number.
• I changed the line with my $content ... to catch the match rather than comparing $content with the pattern.
• $test is not defined anywhere in your code fragement.
• You test for match with $string in the if clause, but then you populate $content based on a different match: /$string/[a-z]*. If the former matches the latter might still fail! This might cause your problem.

# what is in $test ? Not specified.
my $string = quotemeta("$test");

foreach my $filename (@daten){
    print "finding -$string- in $filename\n"."</br>";
    
    open(my $file,  "<",  $filename)  or die "Can't open the file: $!"
+;
    while (<$file>) {
        
        if (/$string/)
        {     
            print; # NEW STATEMENT
            my ($content) = m/(\/$string\/[a-z]*)/g; # different match
+, might fail
            print $content;
            print "found string -$string- in line $. \n"; # $. is the 
+line number
            print "<br>";
        }
    }
}
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UPDATE: added a print statement to check hypothesis

It's usually wise to post a self contained script (along with data) which reproduces a problem. If you're new here PerlMonks for the Absolute Beginner and How do I post a question effectively? are worth reading.

If $string is "a", then \/$string\/[a-z]* does NOT match "air". It would match "/a/ir".

$string = "a";
print "success 1\n" if "air"   =~ /$string/;
print "success 2\n" if "air"   =~ /\/$string\/[a-z]/;
print "success 3\n" if "/a/ir" =~ /\/$string\/[a-z]/;
print "success 4\n" if "air"   =~ /$string[a-z]/;      # UPDATE
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