Aaaarghhhhh. Good catch! Thank you.

next is a left-over (forgetfully 'unremoved') from an alternate approach which was later abandoned.

As to the possibility of multiple variables with similar name_parts -- all on a single line: that's not something I considered, as it appears to be outside the OP's spec. Oversight aside, however, that does indeed make the coding a little tricky...
      ... perhaps something like this [added to prev code and, yes, with the next removed :-)]:

say "\tDone with source and sink files\n";

$/ = "\n";      # restore record separator


my @choroba = ('choroba my car -- a $car -- is $car_colour. Choroba-',
              'choroba my car2 is $car_colour $car.  Choroba-',
              'choroba my car3 is $car_colour $car Choroba-',
              'choroba my car4 is $car_colour and |secondinstance: $ca
+r_colour| |firstinstance:$car| Choroba-',
              );

for (@choroba) {
    if ( $_ =~ /choroba/ .. /Choroba-/ ) {
        s/choroba/ /;           # ...
        s/Choroba-/ /;          # or set record separator
        s/\$car_colour/red/g;   # yup, need the g
        s/\$car[^_]{1}/ford/;
        say $_;
    }
}
[download]

        Done with source and sink files

  my car -- a ford-- is red.
  my car2 is red ford
  my car3 is red ford
  my car4 is red and |secondinstance: red| |firstinstance:ford
[download]

Fun!