The problem is in the implementation of this clause:

  Either INOUT or OUTPUT
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Your regular expression is getting parsed as:

  /
    # either this
    ^ S \s+ [-]* \d+ [\.\d+]* \s+ [-]* \d+ [\.\d+]* \s* \( \s* IOPUT
  |
    # or this
    OUTPUT \s* \)
  /ix
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To achieve your aims, you need to tell perl where your list of alternates starts and ends, with capturing (...) or non-capturing (?:...) parens:

  /
    # all of this
    ^ S \s+ [-]* \d+ [\.\d+]* \s+ [-]* \d+ [\.\d+]* \s* \( \s*
    # and one of these two
    (?: IOPUT | OUTPUT )
    # and all of this
    \s* \)
  /ix
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I'd also recommend using the extended layout permitted by the //x flag for long expressions like this.

Hope this helps,

Hugo