Refer to Parse::RecDescent's <skip> directive:

For the purpose of matching, each terminal in a production is considered to be preceded by a "prefix" - a pattern which must be matched before a token match is attempted. By default, the prefix is optional whitespace (which always matches, at least trivially), but this default may be reset in any production.

That's a convoluted way of saying all productions are implicitly prefixed with a /\s*/ rule. This can be overridden as follows:

...

doc: <skip:""> segment(s)
    { $return =1; }

...
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output
======
got a IEA (~\n)
got a IEA (~\n)
got a IEA (\n)
got a IEA (\n)
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