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`;
    }
}