Depends on if you want modified time, access time or inode change time.

For modified time which is likely the most common.

if( -M "file.pdl" < -M "file.cxx" ) {
  print "file.pdl is newer\n";
}
[download]

Likewise with -A for access time and -C for inode change time ( which used to be create time hence the C )

These filetests list the files age in days since the start of your perl script execution. This is usually positive for a briefly running script if your dates on the filesystem reflect reality and your clock is correct. Thus by comparing 2 numeric quantities you can make these descisions.

update: mea culpa, changed the text 'negative' to bolded 'positive'. Think days ago.