Can't get Perl v5 substr to replace a given selection

[j.smith1981]

Hi there, having a bit of trouble with this substr() function in Perl.

 

This is the code:

#!/usr/bin/perl
print "Content-type: text/html\n\n";

my $string = 'Jeremy climbed up the tree';
my $name = substr($string, 0, 6);
print $name."<br>\n";

my $nickname = substr($string, 0, 6, 'Jez'); ## supposed to output: Jez climbed up the tree not the above original string, but it's still doing the original string

print $nickname;

 

But it's not replacing as in the comments Jeremy with jez on the 2nd use of the substr() function, is there any ideas as to why this is not working?

 

Been having a look at this for 2 months at least and kept going away from Perl expecting to be obvious but it's still doing the same thing.

 

Any help is much appreciated,

Jeremy.