sub pie(&@) {
  my ($cb, $file) = @_;
  rename $file, "$file~"      or die "Error backing up $file: $!";
  open my $IN,  "<", "$file~" or die "Error reading $file~: $!";
  open my $OUT, ">", $file    or die "Error writing $file: $!";

  local $_;
  while (defined( $_ = <$IN> )) {
    $cb->();
    print $OUT $_;
  }
}

pie { s/^\s+// } "myfile.txt";