Unable to install iTunes 12.2
Looking at the install.log you pasted, one line jumped out at me:


  Jul  6 10:46:49 Olly.local installd[661]: preinstall: perl: posix_spawn: /usr/bin/perl5.18: Exec format error


I think that error says that the installer tried to run the preinstall script using the system Perl, but that the system Perl could not even be executed. Could you please run the command file /usr/bin/perl5.18 and tell us what you get? Here's what I get:


  localhost:~ user$ file /usr/bin/perl5.18
  /usr/bin/perl5.18: Mach-O universal binary with 2 architectures
  /usr/bin/perl5.18 (for architecture x86_64):    Mach-O 64-bit executable x86_64
  /usr/bin/perl5.18 (for architecture i386):  Mach-O executable i386


This is a basic hello-world program in Perl; once you get this working on your computer, I think your installer will work fine too.


  localhost:~ user$ /usr/bin/perl5.18 -e 'print "hello, world!\n";'
  hello, world!