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