There are a few strange functions in Perl that can be on the left-hand-side of an assignment. For example if you would like to change the content of a string you can use the 4-parameter version of substr, the 4th parameter being the replacement string, or you can use substr as an Lvalue and assign that string to the 3-parameter version of substr.

substr $text, 14, 7, "jumped from";

and

substr($text, 14, 7) = "jumped from";

make the same changes in $text

Try the examples:

4-parameter substr

use strict;
use warnings;
use 5.010;

my $text = "The black cat climbed the green tree.";
substr $text, 14, 7, "jumped from";
say $text;

3-parameter substr as Lvalue

use strict;
use warnings;
use 5.010;

my $text = "The black cat climbed the green tree.";
substr($text, 14, 7) = "jumped from";
say $text;

They will both print: 

The black cat jumped from the green tree.

Which one to use?

I think the 4-parameter version is much clearer. If you want to make sure others in your team don't use the Lvalue version of substr, you can use Perl::Critic, and make sure the ProhibitLvalueSubstr policy is enabled.