Updated code:

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

my $string = 'This is string';
my $capture1 = substr $string, 6, 2, "is stretched"; # This will subst
+itute as expected

# REALLY?

print $string,"\n";

#  I get:
#  "This iis stretchedstring"

# perhaps better is:
my $string2 = 'This is string';
$string2 =~ s/string/streched string/;
print "$string2\n";

#  that prints:
#  "This is streched string"

# Your second substr():

substr $string, 9, 6, 'STRING';
print $string,"\n";

# that prints:
# This iis STRINGhedstring

# I suspect that you want:
$string2 =~ s/string/STRING/;
print "$string2\n";

# that prints:
# "This is streched STRING"
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Perl is wonderful with what are called "regular expressions" or regex'es. In general, use the power of the Perl language. substr() is a low level thing that can be used, but is very, very rare in Perl programs. substr() is very common in C programs because there is not a "built in regex" operator.

To summarize:

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

my $string = "This is string";
print "$string\n";

$string =~ s/string/stretched string/;
$string =~ s/string/STRING/;

print "$string\n"; 

__END__
This is string
This is stretched STRING
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