A few things:

• It's easier to process the file line by line.
• = is assignment, something you don't want when matching.
• The text of the file is in $slurp, which you then never use.

I'd do something like (untested, but complete program):

@ARGV = "filename.txt";
while (<>) {
    print if /ccParty<.+>DTMF<.+>/ && /_NN/;
}
[download]

Or rather, forget about Perl, and just use a shell one liner (again, untested):

$ grep 'ccParty<..*>DTMF<..*>' filename.txt | grep _NN
[download]

I am assuming your regexp is indeed what you want, I haven't checked whether it matches your description.