my $file = '/path/to/file';
my $line_start = 'BB';
my $search  = 'wort3';

open my $fh,'<',$file or die $!;
while(my $line = <$fh>){
    next unless $line =~ /^\Q$line_start\E/;
    print "yes\n" if $line =~ /\Q$search\E/;
}
close $fh;

use strict;
use warnings;

sub create_file ($$) {
   my ($lines, $file) = @_;
   open my $fh, '>', $file or die $!;
   for (1..$lines) {
      print $fh sprintf("%0".length($lines)."d", $_), " wort1 wort2 wort3\n";
   }
   close $fh;
}

sub search_in_file ($$$) {
   my ($line, $file, $search) = @_;
   open my $fh, '<', $file or die $!;
   my $first_line = <$fh>;
   my $length = length($first_line);
   seek($fh, $length*(--$line), 0);
   my $this_line = <$fh>;
   return 1 if $this_line =~ /$search/;
   return undef;
}

my $lines  = 100000;
my $search = 'wort1';
my $line   = 50000;
my $file   = './test.txt';

create_file($lines, $file);

if (search_in_file($line, $file, $search)) {
   print "HIT\n";
} else {
   print "NO HIT\n";
}