Or if the phrase "negative lookbehind assertion" fills you with dread ...

my $string = 
'Engine4.2.0.5:prefix=PDE:path=C:\h\COE\COMP\PDE:status=installed:vers
+ionNumber=4.2.0.5:type=SOFTWARE-COE:';

$string =~ s/([^C]):/$1'/g;
[download]

...will do it too.

Update: ...almost:( See Tachon's post below for how to do it properly {sigh}.

It reads; capture any single non-"C" char followed by a ":" and replace both by the captured char ($1) and the "'".