Hello, Monks, please shine your wisdom on this doubtlessly elementary problem.

The regular expression in this code sample is designed to work on any line that begins with a tilde character, and in such a line, replace the first underscore with a plus sign.

#!/usr/bin/perl

while (<DATA>)
{
  s/^ ~ .*? \K  _  /+/x;

  print "$_";
}

__DATA__
A line with an_underscore.
A line with_two_underscores.
~A line with an_underscore starting with a tilde.
~A line with_two_underscores starting with a tilde.
[download]

It works exactly as expected, producing the output

A line with an_underscore.
A line with_two_underscores.
~A line with an+underscore starting with a tilde.
~A line with+two_underscores starting with a tilde.
[download]

That is, on the first tilde line, it replaces the only underscore with a plus, and on the second, it replaces only the first underscore, leaving the second one an underscore in the output.

To modify this code so that it changes all underscores in tilde lines to plus signs, the classic method is to add the /g option to the s// operator. But this doesn't work; it produces the same output as above, leaving the second underscore in the last line an underscore rather than changing it also to a plus sign as /g requests. What is going wrong?

In case the \K inside the regular expression was somehow inhibiting the /g option, I rewrote the substitution line to use a capture group instead:

  s/( ^ ~ .*? )  _  /$1+/xg;
[download]

But this exhibits the same failure.

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