To be honest, I stopped using regexp's for this a while ago. I use substr now:

if ($folder_path =~ /^$sourcedir/) {
  $folder_path = File::Spec->catdir('Eitv9',substr($folder_path, lengt
+h $source_dir));
}
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That said, if you want to use regexp's for this, you don't need the "if":

$folder_path =~ s/^\Q$sourcedir/Eitv9/;
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That's because "if" it doesn't match, the substitution will fail (silently - no error message), and $folder_path will be left alone.