I don't see anything wrong with that at all... In fact, when I run it, I get the exact string you're wanting it to create..... 
#!/usr/bin/perl -wT
use strict;

my $new_type = "BookInstructional MaterialTeaching Guide";
print "Before: $new_type\n";

$new_type =~ s/([a-z])([A-Z])/$1::$2/g;

print "After:  $new_type\n";

=OUTPUT
Before: BookInstructional MaterialTeaching Guide
After:  Book::Instructional Material::Teaching Guide

[download]
Update: The double colon is special in perl so if it really isn't working for you, I'd try to disambiguate /$1::$2/ using /${1}::${2}/ as in: 
$new_type =~ s/([a-z])([A-Z])/${1}::${2}/g;

[download]

-Blake

In reply to Re: Finding lowercase letters next to uppercase letters by blakem
in thread Finding lowercase letters next to uppercase letters by dsalada

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