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;
}