c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
 ;;
 use Test::More 'no_plan';
 use Test::NoWarnings;
 ;;
 my $DNA3 = 'ATCGC';
 my $DNA4 = 'AAATTGC';
 ;;
 my $n = 1;
 $DNA3 =~ s{ \A .{$n} \K }{$DNA4}xms;
 ;;
 is $DNA3, 'AAAATTGCTCGC', 'string splice ok';
 ;;
 done_testing;
"
ok 1 - string splice ok
1..1
ok 2 - no warnings
1..2