It might be done with something like substr EXPR,OFFSET,LENGTH,REPLACEMENT:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'abcdefg';
 print qq{'$s'};
 ;;
 my $char = 'X';
 my $n = 3;
 substr $s, $n, 0, $char;
 print qq{'$s'};
"
'abcdefg'
'abcXdefg'
[download]

Update: Changed example code to make position of insertion a variable.