More terser (and check out HTML::LinkExtor, too):

use URI::Escape; # or CGI.pm has an escape() function
# also at the top of the script.
my $pattern = quotemeta( lc( $ARGV[0] ) );

# in your loop
if (my ($mp3) = /^<a href="([^"]+)/) {
    # $mp3 should now hold a URL
    if ($mp3 =~ /$pattern/i) {
      $mp3 = uri_escape($mp3);
      `mpg123 -p none -b 500 -q $mp3`;
    }
}
[download]

Update ChemBoy caught an error (didn't assign to $mp3 in a list context! Check those parens, kids!)

HTH!

perl -e 'print "How sweet does a rose smell? "; chomp ($n = <STDIN>); 
+$rose = "smells sweet to degree $n"; *other_name = *rose; print "$oth
+er_name\n"'
[download]