$!/usr/bin/perl

$dir = "/path/to/files/with/trailing/slash/";

opendir($dh, $dir);

while (readdir $dh) {

    $new_filename = "yourname.txt";

    if (-e "$dir$new_filename") {

        print "Error: file already exists ($new_filename)\n";

    }
    else {

        rename "$dir$_", "$dir$new_$filename";
        print "File $_ renamed to $new_filename\n";

    }

}