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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