perl query using -pie - perl

Question

This works:
perl -pi -e 's/abc/cba/g' hellofile
But this does not:
perl -pie 's/cba/abc/g' hellofile
In other words -pi -e works but -pie does not. Why?

Answer 1

The -i flag takes an optional argument (which, if present, must be immediately after it, not in a separate command-line argument) that specifies the suffix to append to the name of the input file for the purposes of creating a backup. Writing perl -pie 's/cba/abc/g' hellofile causes the e to be taken as this suffix, and as the e isn't interpreted as the normal -e option, Perl tries to run the script located in s/cba/abc/g, which probably doesn't exist.

Answer 2

Because -i takes an optional extension for backup files, e.g. -i.bak, and therefore additional flags cannot follow directly after -i.
From perldoc perlrun
-i[extension]
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, following these
rules:
If no extension is supplied, no backup is made and the current file is
overwritten.
If the extension doesn't contain a * , then it is appended to the end
of the current filename as a suffix. If the extension does contain one
or more * characters, then each * is replaced with the current
filename. In Perl terms, you could think of this as:

Answer 3

perl already tells you why :) Try-It-To-See
$ perl -pie " s/abc/cba/g " NUL
Can't open perl script " s/abc/cba/g ": No such file or directory
If you use B::Deparse you can see how perl compiles your code
$ perl -MO=Deparse -pi -e " s/abc/cba/g " NUL
BEGIN { $^I = ""; }
LINE: while (defined($_ = <ARGV>)) {
s/abc/cba/g;
}
continue {
die "-p destination: $!\n" unless print $_;
}
-e syntax OK
If you lookup $^I in perlvar you can learn about the -i switch :)
$ perldoc -v "$^I"
$INPLACE_EDIT
$^I The current value of the inplace-edit extension. Use "undef" to
disable inplace editing.
Mnemonic: value of -i switch.
Now if we revisit the first part, add an extra -e, then add Deparse, the -i switch is explained
$ perl -pie -e " s/abc/cba/g " NUL
Can't do inplace edit: NUL is not a regular file.
$ perl -MO=Deparse -pie -e " s/abc/cba/g " NUL
BEGIN { $^I = "e"; }
LINE: while (defined($_ = <ARGV>)) {
s/abc/cba/g;
}
continue {
die "-p destination: $!\n" unless print $_;
}
-e syntax OK
Could it really be that e in -pie is taken as extension? I guess so
$ perl -MO=Deparse -pilogicus -e " s/abc/cba/g " NUL
BEGIN { $^I = "logicus"; }
LINE: while (defined($_ = <ARGV>)) {
s/abc/cba/g;
}
continue {
die "-p destination: $!\n" unless print $_;
}
-e syntax OK
When in doubt, Deparse or Deparse,-p