In your case, it looks like you don't want to replace, anyway, but to insert. You could try whether turning your search pattern into a lookbehind might work.

Yes, good insight. It seems I'm using the wrong tool for the job. I've toyed with lookbehinds and never found a good use case that didn't hurt my brain but this seems like the perfect one.

$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

Never used it but looking at the source I'm not sure that it'll do what you want. Your second argument is getting passed in as $replace into this bit of code:

  else {
    @content = split( /\n/, cat($file) );
    for (@content) {
      s/$search/$replace/;
    }
  }
[download]

You've tried to quote things correctly, but for that to work there'd need to be an /ee modifier on the substitution to get it evaluate the contents of $replace and then replace with what that expanded text contains. Unless there's some new dark magic I'm not aware of for expansions that lets you turn on /ee at a distance I don't think it's capable of what you're trying to do.

Edit: Aah you've got the multiline option set; the substitution would be the similar s///mgs a few lines earlier but the problem remains the same (it'd need to be s///mgsee to expand $replace and then evaluate the expanded value).

OK, good to know. My brain seems to have lost all knowledge of the 'e' modifier. Tried to get it to work with this simple example:

my $new = 'blah';
my $string = 'asdfjkjl';
my $find = '(asd)';
my $replace = '$1' . $new;

replace ($string, $find, $replace);

sub replace {
  my $string = shift;
  my $left = shift;
  my $replace = shift;
  $string =~ s/$left/$replace/e;
  print $string;
}
[download]

Still prints $1 in the output though. Adding 'ee' throws an error: "Bareword found where operator expected at (eval 1) line 1, near "$1blah"

$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

You need to set up your replacement string correctly. Once that's done it runs fine with the /ee modifier:

use strict;
use warnings;

use Test::More tests => 1;                      

my $new     = 'blah';
my $string  = 'asdfjkjl';
my $find    = '(asd)';
my $replace = qq#\${1} . '$new'#;

my $result = replace ($string, $find, $replace);

is $result, 'asdblahfjkjl';

sub replace {
    my $string  = shift;
    my $left    = shift;
    my $replace = shift;
    $string =~ s/$left/$replace/ee;
    return $string;
}
[download]

You will probably get a warning, but there is an outside chance that sed qr{("name": "$section",\n.*\n.*)­}, '\\1' . $json, $groups_file, multiline => TRUE might work.

For compatibility, Perl also recognizes '\N' for one-digit N in regex substitutions if there are sufficient capture groups in the pattern.