My usual trick for that is:

#!/usr/bin/perl
$^I = ".bak";
@ARGV = glob "*";
while (<>) {
  s/old/new/ and $changes = 1;
  print;
  if (eof) { # retreat if no changes:
    rename "$ARGV$^I", $ARGV unless $changes;
    $changes = 0; # reset
  }
}
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