If you want to get the combined results of searches for three different subjects, you can run each search individually and build one big list to loop over. Your code might look like this:

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...
}
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There are much cleaner ways to do this (using map with a list of subjects), but hopefully this makes it clearer what is going on.

In general, if you want to know the size of an array, you use the array in scalar context. That is to say:

# sets $num_results to size of @all_results array.
my $num_results = @all_results;
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... which would give you the total number of results your search returned. I'm not sure if this is enough in your case though, as you might need to loop over your results to identify only those dated in the last X minutes.

Hope this gets you started!