Things like this are a lot easier if the data is in a hash .... they you can just find your keys and replace with the hash value. Also, regex (see perlre, especially for details about the /e modifier)is the way to go here instead of the substr string walking. You may also want to see if something like Template Toolkit will quit your needs/be easy to implement in your case.

my %data = (
  ma_test => 1,
  mb_test => 'the RTE',
);
my $text = 'titl=This is test $ma_test of $mb_test broadcasting system
+';
$text =~ s/\$(\w+)/ $data{$1} /g;
print $text;
[download]

Alternatively, you might want something like this:

my %data = (
  ma_test => 1,
  mb_test => 'the RTE',
);
my $text = 'titl=This is test $ma_test blah $x of $mb_test broadcastin
+g system';
$text =~ s/\$(\w+)/ exists $data{lc $1} ? $data{lc $1} : '==VARIABLE_N
+OT_FOUND==' /eg;
print $text;
[download]