Raw Input 
12345678901234567890
Desired Output 
12345678901HERE234567890
Script and Comments
Script1 [sed] 
[ 1] s/^.\{11\}/&HERE/
Script2 [perl] 
[ 1] $string = '12345678901234567890';
[ 2] $string =~ s/^(.{11})/${1}HERE/;
[ 3] print "$string\n";