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my $x = q{Here are [[a variable]] number [[of words]] in brackets};
say $x;
$x =~ s{\[\[(.*?)\]\]}{ (my $x=$1) =~ s/ /_/g; $x }ge;
say $x;

Here are [[a variable]] number [[of words]] in brackets
Here are a_variable number of_words in brackets

sub blank2under {
  my $x=shift;
  $x =~ s/ /_/g;
  return $x;
}

sub rx (&$) {
  my $c_regex=shift;
...
print "$x\n";
$x =~ s(\[\[(.*?)\]\])( rx {s/ /_/g} $1 )ge;
print "$x\n";