... keeps the whole thing inside a single s/// operator.

The only thing I would say about this is that you're firing up the eval-uator behind the scenes, so
    s/^ ~ .* /@{[  ${^MATCH} =~ tr#_#+#r  ]}/xp;
is (I think; haven't tested it) exactly equivalent to
    s/^ ~ .* / ${^MATCH} =~ tr#_#+#r /xpe;
That's an awful lot of moving parts for what seems a fairly simple match and transformation. Getting back to Eily's first solution and in particular to tybalt89's solution, I don't see a problem with something like (also untested): 

while (<FILEHANDLE>) {
    tr/_/+/ if m{ \A ~ }xms;
    do_something_with_fixed_up_line($_);
    }

[download]
Simple, clear, one-step fixup, do whatever you want with the line thereafter.

Oh, and BTW: Please don't use # as a delimiter for regex expressions; I know we're talking TimToady here, but there's no need for perversity!

Give a man a fish:  <%-{-{-{-<

In reply to Re^2: /g option not making s// find all matches by AnomalousMonk
in thread /g option not making s// find all matches by raygun

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.