if(!-w $mp3_file)
      {
        # This shouldn't happen but...
        chmod 0755,$mp3_file;
      }
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Why do you try to make an MP3 file world executable if you can't read it? And why do you make it readable for group and world? Why do you even think about changing the mode of files that are not writeable?

There are usually good reasons for chmod -w somefile and chmod go-rw somefile: I don't want to accidentally modify a file in the first case, and I don't want others to read my files in the second case. Your hardcoded chmod 0755 ignores both. To make things worse, it sets the executable flag, possibly creating a new vulnerability.

I won't start with race conditions between stat (implied in -w) and open, because MP3::Tag has the same problem, so your code does not make it worse in this special case. Generally, one should not try to predict if open fails or succeeds by using stat. It can't work reliably (race condition), and it is not needed. open sets errno a.k.a. $! on error, and errno delivers sufficient information why open failed.

Alexander