$$text_ref =~ s%
  (STARTPRESERVE1.*?STOPPRESERVE1)
|
  (STARTPRESERVE2.*?STOPPRESERVE2)
|
  (PRESERVESTART.*?PRESERVESTOP)
|
  \s+
%
  ((defined $1 and ($1 =~ /^\s+$/)) ? ' ' : $1) .
  ((defined $2 and ($2 =~ /^\s+$/)) ? ' ' : $2) .
  ((defined $3 and ($3 =~ /^\s+$/)) ? ' ' : $3)
%eigsx;