in reply to Tricky regex problem

If that (update: oops... I meant to reply to that node) works for you, try this as a simplification (untested): 
$input_text =~ s{
    ^ ([#]{2,5}) \s+ [^\n]+ \s+ (?= ^ \1 [^#])
    }
    {}xmsg;

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Beyond that, I have to say I don't understand your requirements. Can you express them more clearly?

Give a man a fish:  <%-{-{-{-<

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Re^2: Tricky regex problem (updated)
by nysus (Parson) on Jul 22, 2020 at 15:47 UTC

    All headers that are followed by only whitespace, with the whitespace getting followed by a header with the same or fewer pounds signs as the initial header, should get stripped.

    $PM = "Perl Monk's";
    $MCF = "Most Clueless Friar Abbot Bishop Pontiff Deacon Curate Priest Vicar";
    $nysus = $PM . ' ' . $MCF;
    Click here if you love Perl Monks

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