stewartski,
A quick cursory glance at your code doesn't reveal any immediate problem. So now I can only offer you possibilities.

• See if it behaves the same way on a different server
• See if it behaves the same way in active mode
• Change $ftp->ls() to $ftp->dir() and manually parse the time stamp
• Modify the mdtm sub from Net::FTP and possibly provide a bug fix.

I will explain the last part now. You want to temporarily change the mdtm sub to the following:

sub mdtm {
    my $ftp  = shift;
    my $file = shift;
    return $ftp->_MDTM($file) || undef;
}
[download]

This won't fix anything - this is strictly to see what the stamp looks like. If you can subsequently modify the original code to properly parse it great - send in a patch. If not, post a new SoPW question saying how can I convert "X" into epoch time stamp.

Cheers - L~R