It's faster to use index() when looking for exact matches. For example, I would change your first section to this: 
    # Only care about comments
    next unless ( index( $_, '%%' ) == 0 );
    # Duplex or no...
    if (( index( $_, '%%' ) == 0 && index( $_, 'Duplex Duplex' > -1 ))
          ||
      /^[&l1S/i
          ||
      /^[&l2S/i
          ||
      index( $_, 'DUPLEX=ON') > -1 ) {
        $pagestate = 'Duplex';
    }

[download]
You should make sure that the most common situations are the ones that come first in that list of || conditions. Also, for large files it can be helpful to read in chunks bigger than one line. I found this node sped things up a lot when I was doing something similar.

In reply to Re: Fast file parsing by perrin
in thread Fast file parsing by Anonymous Monk

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