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;