use strict;
use warnings;
my $str="1 2 3 4 5 6 7 8 9 10";
print "string is $str\n";
$str=~s/(?<=(\d\s){1})\d*/I am replaced/;
print "string is $str";

##</code><code>##

$str=~s/(?<=(\d\s){0})\d*/I am replaced/;