I found an example at the Perl-Advent-Calendar. It uses unix-time in seconds. I've changed my script and it works fine now.

#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;
use Date::Calc qw/ Time_to_Date /;
use File::Find::Rule;

my $yesterday = time()-(1*24*60*60);
print $last_week, "\n";
my @files = File::Find::Rule->file()
                            ->name( '*.doc' )
                            ->mtime( ">$yesterday" )
                            ->in( 'D:/Dokumentation/' );

@files = map {
  {
    name => $_,
    #modified => [Time_to_Date((stat($_))[9])],
    size => (stat($_))[7]
  }
} @files;

print Dumper \@files;
[download]

Thanks in advance, neniro