Different definition of "in place".

Per perlrun, setting $^I is like including the -i option on the command line. From perlrun:

specifies that files processed by the <> construct are to be edited in-place. It does this by renaming the input file, opening the output file by the original name, and selecting that output file as the default for print() statements. The extension, if supplied, is used to modify the name of the old file to make a backup copy

So setting $^I = '.bak' has the same effect as my code. Minor difference: My code reads the original, writes the new, then renames both files. Setting $^I causes the original to be renamed first.

The "in place" I was referring to is, literally, modifying the existing file where it is stored1. This can be done through careful use of tell and seek. The following should work when replacing existing bytes:

open my $fh, '<+:raw', $file; # raw so file is read/written in bytes, 
+not characters
my $where = tell($fh);
while (<$fh>) {
    ...; # modify 1 or more bytes
    seek $fh, $where, SEEK_SET;
    print $fh, $_;
    $where = tell($fh);
}
close $fh;

[download]

If attempting to add or delete bytes, more work needs to be done.

---

1 This ignores the possibility that the underlying storage manager/controller could write the modified blocks of the file to different locations in storage, then update the file's "block map" after the write is successfully completed.

In reply to Re^3: Trying to substitute a variable with another variable in a string but not working by RonW
in thread Trying to substitute a variable with another variable in a string but not working by kaushik9918

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