perl query using -pie

Question

This works:

perl -pi -e \'s/abc/cba/g\' hellofile

But this does not:

perl -pie \'s/cba/abc/g\' hellofile

Answer 1

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 { $^I = ""; }
LINE: while (defined($_ = )) {
    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 { $^I = "e"; }
LINE: while (defined($_ = )) {
    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 { $^I = "logicus"; }
LINE: while (defined($_ = )) {
    s/abc/cba/g;
}
continue {
    die "-p destination: $!\n" unless print $_;
}
-e syntax OK

When in doubt, Deparse or Deparse,-p