What follows is a solution which requires the minimum knowledge of the format. It works with the two special cases. Sorry, I must be tired today.

while (<>) {
   chomp;

   while (
      /
         (\w+)                      # An identifier.
         \s* = \s*                  # Equal with opt spaces.
         (
            (?:
               (?! \s+ \w+ \s* = )  # Stop if we see the next formula.
               .                    # A chararacter.
            )+
         )
      /xg
   ) {
      my ($name, $expr) = ($1, $2);

      $expr = substr($expr, 1, -1)
         if substr($expr, 0, 1) eq "'";

      print("var: $name, expr: $expr\n");
   }
}
[download]

Output:

var: drsubc, expr: agauss(0, 1, 3)        <- Works
var: delm1, expr: 0 + 0.045u*distm1
var: delm2, expr: 0 + 0.07u*distm2
var: delm3, expr: 0 + 0.07u*distm3
var: delm4, expr: 0 + 0.07u*distm4
var: delmt, expr: 0 + 0.07u*distmt
var: delml, expr: 0.16u + 0.43u*distml
var: delam, expr: 0.32u + 0.86u*distam
var: dele1, expr: 0 + 0.25u*diste1
var: dele2, expr: 0 + 0.25u*diste2
var: delma, expr: 0.16u + 0.6u*distma
var: pmsxt, expr: npmsxt + 12.5u*dpmsxt
var: tih, expr: 0.35u                     <- Works
var: capct, expr: 0.50u + 0.13u*xdcapct
var: capcti, expr: 0.55u + 0.13u*xdcapct
var: m1t, expr: 0.41u + 0.05u*xdm1t
var: m1ti, expr: 0.36u + 0.05u*xdm1t
var: m2t, expr: 0.48u + 0.057u*dm2t
var: m3t, expr: 0.48u + 0.057u*dm3t
var: m4t, expr: 0.48u + 0.057u*dm4t
var: mtt, expr: 0.48u + 0.057u*dmtt
var: qtt, expr: 0.242u + 0.0202u*dqtt
var: htt, expr: 0.242u + 0.0202u*dhtt
var: mlt, expr: 2.0u + 0.2u*dmlt
var: amt, expr: 4.0u + 0.4u*damt
var: e1t, expr: 3.0u + 0.5u*de1t
var: e2t, expr: 4.0u + 0.5u*xde1mat
var: mat, expr: 4.0u + 0.4u*dmat
var: m1m2t, expr: 0.35u + 0.05u*dm1m2t
[download]

hmmm.... (?! xyz ) { what-bang? :) }

New one on me, but that's an excellent idea.

Stop when you see xyz...

Perl++ never ceases to amaze me.

How do I say 'anything including spaces up to the first occurrence of more than one space in a row'?

A literal translation (untested) would be /(?>.*?(?=  ))/s.

Here's an alternative, in case yours doesn't work:

# Read these comments from the bottom up.
/
   (
      (?:
         (?! \s{2} )  #    which aren't the start of 2 spaces.
         .            #    characters
      )*              # zero or more
   )                  # Capture
/sx
[download]

Sorry - this doesn't handle the non-quoted element.

I fixed it while you were replying :)