This is covered in perlfaq5 under the topic: How do I get a file's timestamp in Perl? There's enough text there that it's probably preferable for you to visit that link rather than have someone paste the contents here.

See also the -M file test operator, and the File::stat module (both are discussed in the perlfaq referenced above). File::stat is part of the Perl core, so shouldn't be considered an external dependency.