As Discipulus pointed out, using both -p and -n doesn't make much sense. And you'll have to use the correct operator as Corion told you

Your issue is that -p prints $_ no matter what, it doesn't print back the splitted data of @F. You have to do that yourself: $" = qq(\t);$_="@F"; You'll find $" in perlvar.

Edit: you can try this to see the code that perl generates from your one liner: perl -MO=Deparse -pae '$_ = @F'

LINE: while (defined($_ = <ARGV>)) {
    our(@F) = split(' ', $_, 0);
    $_ = @F;
}
continue {
    die "-p destination: $!\n" unless print $_;
}
[download]

Edit: replaced $"="\t"; by  $" = qq(\t); because the first one looked like a syntax error.

Brava Eily!

never tought to use -MO=Deparse to inspect a oneliner! this trick push me even more up in the dark oneliner side of the source..

Anyway -p seems to superseed -n in both order, results are equal:

perl -MO=Deparse -p -n -e 1
LINE: while (defined($_ = <ARGV>)) {
    '???';
}
continue {
    die "-p destination: $!\n" unless print $_;
}
-e syntax OK

perl -MO=Deparse -n -p -e 1
LINE: while (defined($_ = <ARGV>)) {
    '???';
}
continue {
    die "-p destination: $!\n" unless print $_;
}
-e syntax OK'
[download]

L*

There are no rules, there are no thumbs..
Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.

You're using the wrong operator. You are assigning to $F[0] instead of using regex binding.

perl -pi -w -lane '$F[0] =~ s/AAA/BB/;' *.txt
[download]

could work.

I did try this originally, but unlike '=', '=~' fails to replace anything at all unless -w and -pi are removed - but that then prevents me from batch processing all .txt files.

hello, are not -p and -n togheter, a nonsense?

L*

There are no rules, there are no thumbs..
Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.