Here's a solution that doesn't depend on the order of the lines. It finds all the titles using a regex and counts them using a hash, and then selects the one that appears exactly once, warning or dieing if there isn't exactly one. You haven't specified a few things about the title: line, like whether there can be #'s in the titles, and what kind of text might appear after the closing #.

use warnings;
use strict;

my %mycorpus = (
    a => "<blah blah blah 
blah
title:#this is text I want 1#
blah blah blah",

    b => "blah
title:#this is text I do not want#
title:#this is text I want 2#
blah
title:#this is text I do not want#
blah",

    c => "blah blah
title:#this is text I do not want#
title:#this is text I want 3#
title:#this is text I do not want#
title:#this is text I do not want#
blah",
);

for my $filename (sort keys %mycorpus) {
    my %titles;
    $titles{$1}++
        while $mycorpus{$filename} =~ m{ ^ title:\# (.*) \# }xmg;
    my @once = grep { $titles{$_}==1 } sort keys %titles;
    die "No title found in $filename" unless @once;
    warn "More than one title found in $filename" if @once>1;
    my $title = $once[0];
    print "$title\n";
}

__END__

this is text I want 1
this is text I want 2
this is text I want 3
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