my $foo = "12342343432424232";
my $bar = "substr12342343432424232,3,3";

$foo =~ s/(substr($foo,3,3))/fnord/;
$bar =~ s/(substr($foo,3,3))/fnord/;

print "foo: $foo\n";
print "bar: $bar\n";

##</code><code>##

foo: 12342343432424232
bar: fnord