use strict;
use warnings;

sub toy_sed_on_file {
  my $pattern = shift;
  my $replacement = shift;
  my $file = shift;

  my $count = 0;

  open my $in, '<', $file or die "open $file: $!";
  open my $out, '>', $file.$$ or die "open output ${file}$$: $!";

  while (<$in>) {
    $count += s/$pattern/$replacement/g;  # does nothing if m/$pattern/ would not match
    print $out $_;
  }

  close $in or die "close $file: $!";
  close $out or die "close output ${file}$$: $!";

  if ($count > 0) { rename $file.$$, $file or die "replace input file: $!" }
  else            { unlink $file.$$ or die "remove output file: $!" }

  return $count;
}