Suggestions on your original code:

• $ident needs a newline after you print it.
• /$ident/ needs to be /$ident/o to prevent the (slow) recompiling of the regex on each loop.
• You should chomp the filename, just like you do for $ident.
• If the open fails on either of your files, then the rest of your program cannot run sensibly, so change warn into die.
• use strict and use warnings.
• In your FOUT warning, you used $i instead of $!. 'use strict' would catch that!

The example code below creates a spreadsheet with two rows, "Harry James Potter" and "Amelia Susan Bones", highlighting "James" and "Amelia".

Working, tested code:

use strict;
use warnings;
use Spreadsheet::WriteExcel;

my $out_file = 'result.xls';
my $search_string = 'me';

my $search_pattern = quotemeta($search_string);

my $workbook = Spreadsheet::WriteExcel->new($out_file)
  or die "Can't create new workbook '$out_file': $!";

my $worksheet = $workbook->add_worksheet()
  or die;

my $highlight = $workbook->add_format();
$highlight->set_bg_color('yellow');

my $row = 0;
while (<DATA>) {
  chomp;

  next unless /$search_pattern/o;

  my $col = 0;
  my @fields = split;
  foreach my $field (@fields) {
    if ( $field =~ /$search_pattern/o ) {
      $worksheet->write_string($row, $col, $field, $highlight);
    }
    else {
      $worksheet->write_string($row, $col, $field);
    }
    $col++;
  }
  $row++;
}

$workbook->close()
  or warn "Error closing file '$out_file': $!";

__END__
Harry James Potter
Albus Percival Wulfric Brian Dumbledore
Cornelius Oswald Fudge
Amelia Susan Bones
Dolores Jane Umbridge
Percy Ignatius Weasley
[download]