# Assumes no non-mp3 regular files in the dir
use File::Basename;
for (glob('/path/to/*')) {
    -f or next;    # don't mess with directories, etc
    my ($file,$dir,$ext) = fileparse($_,qr/\.[^.]+$/);
    $file =~ tr/_/ /;
    $file =~ s/(\w+)/ucfirst $1/eg;
    rename $_, $dir.$file.'.mp3' or warn 'Failed to rename ',$_,': ',$!;
}