I decided to try an older perl and installed 5.20.3 but the problem persisted.

Perhaps, but the problem is not with that particular subroutine.

$ cat 1204426.pl 
sub substitute_ac_subst_variables
{
  my ($text) = @_;
  $text =~ s/\${([^ \t=:+{}]+)}/substitute_ac_subst_variables_worker (
+$1)/ge;
  return $text;
}
$ perl -cw 1204426.pl
1204426.pl syntax OK
$ perl -v

This is perl 5, version 20, subversion 3 (v5.20.3) built for x86_64-li
+nux-thread-multi
(with 16 registered patches, see perl -V for more detail)

Copyright 1987-2015, Larry Wall

Perl may be copied only under the terms of either the Artistic License
+ or the
GNU General Public License, which may be found in the Perl 5 source ki
+t.

Complete documentation for Perl, including FAQ lists, should be found 
+on
this system using "man perl" or "perldoc perl".  If you have access to
+ the
Internet, point your browser at http://www.perl.org/, the Perl Home Pa
+ge.

$
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