in reply to Parse C-like define statements

Tough problem. You could try a queue - the idea is to store those keys whose values are not complete, and keep substituting until they are complete: 
use strict;
use Data::Dumper;

my (%hash,@queue);

while (<DATA>) {
   chomp;
   if (/^#DEFINE\s+<([^>]+)>\s+(.*)/) {
      $hash{$1} = $2;
   }
}

do_it($_) for keys %hash;
do_it(shift @queue) while @queue;

sub do_it {
   my $key = shift;
   $hash{$key} =~ s/<([^>]+)>/$hash{$1}/g;
   unshift @queue, $key if $hash{$key} =~ /</;
}

print Dumper \%hash;

__DATA__
------------------
#DEFINE <PATH> /path/to/something
#DEFINE <VERSION> v12<REV>
#DEFINE <REV> 3
#DEFINE <FILE> <PATH>/foo_<VERSION>.txt

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BUT!!! This is going to loop infinitely if you have one single circular reference in your #DEFINE statements. For example, change <VERSION> to: 
#DEFINE <VERSION> v12<REV><FILE>

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and this will not work. Hopefully someone else will have a better answer, but if you are %110 certain that this will not be the case, then this code will prevent unecessary iterations.

p.s. this might be a job for Parse::RecDescent ...

UPDATE: danger++ and sfink++

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)

In Section Seekers of Perl Wisdom