no warnings 'layers';
use open ':locale';
use warnings 'all';

##</code><code>##

perl -pi.bak -e "if( $. == 2 ) { $_ = q/no warnings 'layer';\n use open ':locale';\nuse warnings 'all';\n$_/;}" myscript.pl

##</code><code>##


no warnings 'layer'; # This squelches any warning due
             # to the open pragma not being able to find
             # a locale definition in your environment
             # variables.
use open ':locale'; # set a default locale for all IO
                    # in your script based on env vars.
use warnings 'all';