Hi hector89,

The problem is that readdir returns the names of files (or directories) in the specified directory, but not their paths. So you need to either:

chdir $dirName;
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before entering the foreach loop, or else add:

$txtfile = $dirName . '\\' . $txtfile;
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at the start of the code within the loop to prepend the directory path to each filename.

You probably would have seen this yourself if you had included:

use warnings;
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at the start of your script.

Here is a working version with a little sanity checking added:

#!/usr/bin/perl
use strict;
use warnings;

(my $prog_name = $0) =~ s| ^ (?: .* [\\/])? (.*) \.pl $ |$1|x;

(my $dirName = $ARGV[0])
    or die "Usage: $prog_name <directory>";

opendir(DIR, $dirName)
    or die "Unable to open directory '$dirName': $!";
my @files = grep(/ \.txt $ /x, readdir(DIR));
closedir(DIR);

chdir $dirName;     # <-- EITHER add this

foreach my $txtfile (@files)
{
  # $txtfile = $dirName . '\\' . $txtfile;     # <-- OR add this

    print "$txtfile\n";

    open(my $input_fh, '<', $txtfile)
        or die "Unable to open file '$txtfile' for reading: $!";

    undef($/);
    my $file = <$input_fh>;

    close $input_fh;

    $file =~ s/(<%--)?\s*<script(.*?)>/($1 ? '' : "\n<%-- ") . "<scrip
+t$2>"/ige;
    $file =~ s=</script>\s*(-->)?='</script>' . ($1 ? '' : " -->\n")=i
+ge;

    print $file;

    open(my $output_fh, '>', $txtfile)
        or die "Unable to open file '$txtfile' for writing: $!";
    print $output_fh $file;
    close $output_fh;
}
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HTH,