[Moved parent from CUFP to SoPW. Perl questions are to be posted in SoPW.]

The command (including $$) is being parsed and interpreted by the shell. If you want Perl to interpret $$, it needs to be in the Perl code. Fortunately, inplace editing (-i) can be initiated from within a script.

perl -pe 'BEGIN { $^I = ".$$" } s/search/replace/g' *.html
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Ref: $^I

The remark done by ikegami becomes obvious if you peek the code behind the one-liner with B::Deparse:

$ perl -MO=Deparse -i.$$ -pe 's/search/replace/g' *.html
BEGIN { $^I = ".2412"; }
LINE: while (defined($_ = <ARGV>)) {
    s/search/replace/g;
}
continue {
    print $_;
}
-e syntax OK
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In this case, the "2412" came from the shell as ikegami pointed. I tried to hide $$ from the shell by quoting, but to no avail. Indeed, $$ must be in the Perl code — the argument of -i is interpreted as literal string in attempts like:

$ perl -MO=Deparse -i'.$$' -pe 's/search/replace/g' *.html
BEGIN { $^I = ".\$\$"; }
...
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I would like to have PROCESS_ID of one line perl code as the backup file extension.

One supposes that you are already aware that process IDs get re-used, so that this could easily lead to some of the files being overwritten. It scarcely seems necessary to mention that, since of course you surely were already aware of it, but there I go again, stating the obvious.

UpdateThis was already covered in depth by the rest of the thread, it would be safe for you to ignore it.

I think the reason you're geting the shell's pid is that the shell gets to the $$, sticks it's pid into there, and passes it's pid off to perl as the value of the argument to -i...

If I ask bash,

:~$ echo -i.$$
-i.27383
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I think this was covered above, but i can't be sure.

HTH