First, you need to have "use warnings;" and "use strict;" on all your programs. This will save you a lot of grief.

Before you do somthink like $cftp->mdtm($filename) you have to actually have a filename. E.g.

for my $filename ($ftp->ls) {
  my $filetime = $ftp->mdtm($filename)
     or die "mdtm failed ", $ftp->message;
  # here check if filetime is in your range
}
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