use strict;
use IO::File;

my $in  = IO::File->new;
my $out = IO::File->new;
my $change_on  = "FChangeBar Yes";
my $change_off = "FchangeBar No";  # hmmm, lower case c?


$in->open('test.mif') or die "can't read input";
$out->open('>tmp.mif') or die "can't write output";

while (<$in>) {
   s/$change_on/$change_off/g;
   print $out $_;
}

$in->close or die "can't close input";
$out->close or die "can't close output";

rename('tmp.mif','test.mif') or die "can't rename";