my @results1 = values %{imap->... 'SUBJECT', 'Subj1' ...};
my @results2 = values %{imap->... 'SUBJECT', 'Subj2' ...};
my @results3 = values %{imap->... 'SUBJECT', 'Subj3' ...};
my @all_results = (@results1, @results2, @results3);

for my $h (@all_results){
   # Your code manipulating $h as before...
}

##</code><code>##

# sets $num_results to size of @all_results array.
my $num_results = @all_results;