Add Perl titlecasing

[NWilson]

This is a feature in Perl, which combines the functionality of the replacement casing operators:

$thevar = "Hello";
$thevar =~ s/(Hello)/>\l\U${1}</g;
print $thevar

PCRE2 should support this, for Perl compatibility.

[PhilipHazel]

Good point. I will merge this once all the checks are complete, and I will then update the documentation.