I tried this:

#! /usr/bin/perl

use strict;
use warnings;
use diagnostics -verbose;

use PPI;

my $perlcode = << "END_PERLCODE";
package My::L10N::en;

use base 'My::L10N';

our %Lexicon = (
    'Some [_1] text' =>
    'Some [_1] text' 
    
    'The [_1] is in the [_2]' =>
    'The [_1] is in the [_2]'
);

END_PERLCODE

my $doc = PPI::Document->new(\$perlcode);
my $lexicon = $doc->find(sub { $_[1]->content eq '%Lexicon' });
$lexicon = $lexicon->[0]; # returns an array ref, but in my case, ther
+e's only ever one.

my $newmsg = q['The [_1] is in the [_2]'];
my $newtext = sprintf('    %s =>\\n    %s,', 
              $newmsg, $newmsg, $newmsg, $newmsg);
my $insert_this = PPI::Document->new(\$newtext);

$lexicon->insert_after($insert_this->tokens());

print $doc->serialize;

1;
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