use strict;
my $a;
my $from = 2;
my $len  = 2;

while( $a = <DATA> ) {
	chomp $a;
	print "Before: '$a' ";
	$a = $1 . $2 if $a =~ /^(.{$from}).{$len}(.*)$/i;
	print "After '$a'\n";
}

__DATA__
12345678
yellow
a
xx
yyy