Perl is very flexible with its conversions between numbers and text so there aren't any conversions to ASCII needed:

my $x  = 0b01001001;
my $hi = ($x & 0b11110000) >> 4;
$hi = $hi . " (four)";

print "The answer was $hi";

#The answer was 4 (four)
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As far as a book recommendation goes, I learnt Perl mostly from The Perl Cookbook. A great book if you have some experience of other languages.

Perl comes with a huge amount of help for learning Perl. If you're running Activestate Perl, you should have all the documentation in html format already installed. Have a poke around in the Start menu. Of course, PerlMonks has a wealth of information too. Have a look in the tutorials section.

If you're going to spend any time here at PM, I think you should read Writeup Formatting Tips ;)