among things discussed on p5p, I remember it was mentioned that some RedHat distributions go with DEBUG-enabled builds of perl, which is rather sad.
in reply to Is your Perl running as fast as possible?
In order to get faster build, I tend to turn OFF threading support, which is bad in Perl anyways, and, secondly, I tend to use Perl's memory allocator, which, in turn, is quite a helper and is seemingly better than Tcl's allocator (so when I manage for both Perl and Tcl to use Perl's memory allocator, I see some additional speedup)
As for dramatic speed improvements, related to your XML parsing, it is quite possible for Perl to use or not use Pure-perl and XS parts of module, so check whether your slower installation lacks some binary module.
You may even copy perl between different OSes and see if this result in differences
As a final note, Tcl::Tk is twice faster compared to Perl/Tk