use strict;
use warnings;

sub subsitute {
  (my $filename, my $oldword, my $newword) = @_[0..2];

  local $/;

  if(open(my $fh, "<", $filename)){
     my $contents = <$fh>;

     $contents =~ s/\b$oldword\b/$newword/ig;

     close($fh);

     if(open($fh, ">", $filename){
        print $fh $contents;
        close($fh);
        return 1; # for success
        }
     else{
        warn "Could not open $filename for writing because $!; skipping it\n";
        return undef;
        }  
   }
  else {
   warn "Could not open $filename because $!; it will be skipped\n";
   return undef;
 }
}