How about something like: 

my $file_name = ...;

my $file;
{
   open(my $fh, '<', $file_name)
      or die("Unable to open input file: $!\n");
   local $/;
   $file = <$fh>;
}

my ($P) = $file =~ /A(.*?)B/
   or die("Unable to find 'P'\n");

$file =~ s/(C.*?)\Q$P\E/$1Q/g;

{
   open(my $fh, '>', $file_name)
      or die("Unable to open output file: $!\n");
   print $fh $file;
}

[download]

Update: Added \Q...\E.

In reply to Re: Second Time Replace Regex by ikegami
in thread Second Time Replace Regex by Anonymous Monk

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