I can't reproduce this problem (%users seems to have the right value in the function) but there is a problem with the following:

   foreach(keys %users) {
       my %user=$_;
       [...]
   }
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$_ isn't a hash so it doesn't make sense to assign it to a hash variable. $_ is one of the hash keys returned by keys. What you want is this:

   foreach my $username (keys %users) {
      #my $pw=getpwnam($users{$username}{uname});
      # actually it's simpler, you already have the name...
      my $pw=getpwnam($username);
      my $homedir=$pw->dir;
   }
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I also suggest that your build_quota_list return a reference to the hash it builds and find_deletion_candidates take this reference as input. Otherwise you are making a copy of the whole contents every time you pass it around.