you do a regex on your filehandler and not it content :)
try

#!/usr/bin/perl

use strict;
use warnings;

     print "searching using regexp...";
     
open(my $in,  "<",  "DirectX.log")  or die "Can't open DirectX.log: $!
+";
     
while (<$in>) { # assigns each line in turn to $_
   print "Just read in this line: $_";
   if ($_ =~ /eroare/) {
     print "found eroare on line nr: $.\n";
   }
}

close $in or die "$in: $!";
[download]

thank you!!!!

#!/usr/bin/perl

use strict;
use warnings;

     print "searching using regexp...\n";
my $a = "start";
open(my $in,  "<",  "DirectX.log")  or die "Can't open DirectX.log: $!
+";

#print $a;
while (<$in>) { # assigns each line in turn to $_
   #print "Just read in this line: $_";
   $a = $a . $_;
}

if ($a =~ /eroare+/) {
    print "found eroare\n";
}

close $in or die "$in: $!";
[download]

Now you read the whole file into $a (which can be done more efficiently) but what you want is something like

while (<$in>) { # assigns each line in turn to $_
   #print "Just read in this line: $_";
   if( /eroare/ ) { # checks $_
      print;
   }
}
[download]

check the while loop usage:

  while(my $line=<$in>){
   if ($line=~/eroare/i){
      print "Found";
   }
 }
[download]

If you tell me, I'll forget.
If you show me, I'll remember.
if you involve me, I'll understand.
--- Author unknown to me

readline, perlvar#$.

while ( readline $fh ){
   if( /shabba/ ){
      print "$.:$_\n";
   }
}
[download]