use strict;
use warnings;
my $start_tag  = qr/\*\*\* begin file \*\*\*/;
my $end_tag    = qr/\*\*\* end file \*\*\*/;
my $case = '';

while ( <DATA> ) {
	# The range operator is sorely underused.
	if ( /$start_tag/ .. /$end_tag/ ) {
		if ( /$start_tag/ or /$end_tag/ ) {
			s/file/substituted file/;
		} else {
			$case = $_ & ' ' x length;
			s/old/NEW/i;
		}
		print $_ | $case;
	}
}

__DATA__
test
*** begin file ***
old_word
OLD_WORD
oLd_wOrd
olD_worD
*** end file ***
test2

##</code><code>##

*** begin substituted file ***
new_word
NEW_WORD
nEw_wOrd
neW_worD
*** end substituted file ***