Take a look at stat. All the --something operators are just frontends for this function. UN*X keeps track of last access, last modification, and last inode change (often confused with creation time).

-M looks at the mtime (modification time), and subtracts $^T (see perlvar). A better choice might be -C, which looks at ctime (inode change time). Still, that too just looks at the last time you changed something about the file "metadata": creation, permissions, and the like.