grep is not the best choice for finding a single match in a large array; unfortunately Perl doesn't offer a function that is as simple as grep for this, so people use it anyway.

Depending on the data, a for loop that bails out at the first match is usually faster. Since in this case we want to avoid the readdir calls though, a while is best:

opendir(DIR, $path) or die("No opendir on $path\n");
while (defined($_ = readdir DIR)) {
  if (/^${id}\_.*$/o) {
    $filename = "$path/$_";
    last;
  }
}
closedir(DIR);
if ($filename) {
  unlink($filename) or die("No unlink on $filename\n");
}
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