#!/usr/bin/perl
use strict;
use warnings;

my $string = 'This is string';
my $capture1 = $string, 6, 2, "is stretched"; # This will substitute as expected
my $capture2 = $string, 9, 6, 'STRING'; # Now here original string will be updated and positions I have already are of no use.

__END__
Useless use of a constant (6) in void context at C:\test.pl line 6.
Useless use of a constant (2) in void context at C:\test.pl line 6.
Useless use of a constant ("is stretched") in void context at C:\test.pl line 6.
Useless use of a constant (9) in void context at C:\test.pl line 7.
Useless use of a constant (6) in void context at C:test.pl line 7.
Useless use of a constant ("STRING") in void context at C:\test.pl line 7.

Process completed successfully