Adding just another thought.

As you are mixing basically two different ways of dealing with date&time (Date::Manip and Perl core's localtime), I guess there are also real chances that the two might disagree on what the local timezone is (and as such, how they would deal with the epoch).

Date:Manip says that
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Date::Manip must be able to determine the timezone the user is in.  
It does this by looking in the following places:

   $Date::Manip::TZ (set with Date_Init or in Manip.pm)
   $ENV{TZ}
   the unix ‘date‘ command (if available)
   $main::TZ
   /etc/TIMEZONE
   /etc/timezone

At least one of these should contain a timezone 
in one of the supported forms.  
If none do by default, the TZ variable must be set with Date_Init.
[download]

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Seems quite possible that the two might disagree - which would make 'last sunday' - which is taken at midnight - easily become 'Saturday' even for a slighty different TZ. (If so, it might be that 'last sunday noon' wouldn't have run into troubles).

I'm really curious now about what you'll find.