As noted above, the tricky part is defining the word regex. This should also probably be defined separately and passed to the emphasize() function rather than being hard-coded.

perl -wMstrict -le
"{  my %emphasis;
    my $word = qr{ (\b \w+ \b) }xms;
    sub emphasize {
      my $string = shift;
      %emphasis = map { $_ => 0 } @_;
      my $intro = qq{@{[ sort { $a <=> $b } keys %emphasis ]}};
      my $words = 0;
      $string =~
        s{ ($word) }
         { exists $emphasis{++$words} ? qq{**$1**} : $1 }xmsge;
      return qq{$intro $string};
      }
 }
 my $string = 'Sam goes to school to play football.';
 print emphasize($string, 4, 7);
 print emphasize($string, 7, 2, 4);
 print emphasize($string);
 print emphasize('the cow jumped over the', 3, 4, 1);
"
4 7 Sam goes to **school** to play **football**.
2 4 7 Sam **goes** to **school** to play **football**.
 Sam goes to school to play football.
1 3 4 **the** cow **jumped** **over** the
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