Given a character offset, you can use substr to pick out the part of the string that comes before the word in question. Given that string, you can count the words, perhaps by splitting it. Given the substr that comes after the character offset, you know the word is at the front. You can use a regular expression to pull out that word. Then you can get its length to see what of the string comes after it. That should give you enough info to know where to put the asterisks before you print the result.

#!/usr/bin/perl
use warnings;
use strict;

my $sentence = "Sam goes to school to play football";
my $word = substr($sentence,12,17-12+1);
my $next_word = substr($sentence,27,34-27+1);
my $find_word = word_counter($sentence);
print "$word: ", $find_word->($word),"\n";
print "$next_word: ", $find_word->($next_word),"\n";

sub word_counter {
    my $sentence = shift;
    my @words = split " ",$sentence;
    return sub {
        my $word = shift;
    my $count = 0;
    for (@words) {
        $count++;
        return $count if ( $word eq $_ );
    }
    return 0;
    }
}
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C:\Code>perl stuff1.pl
school: 4
football: 7
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There are a couple of things to think about here. You need to figure out how you are going to store the word spans (where do they come from btw?). Deciding just what is a word can be tricky (e.g. for example).

When editing involves changing the length of whatever is being edited it is often better to work backwards so that you don't change the indexes for subsequent edits. The following somewhat golfed code (don't hand it in to your teacher) counts words by counting the spans of spaces between them:

use strict;
use warnings;

my $sentence = 'Sam goes to school to play football.';
my @subs = ([12, 17], [27, 34]);
my @counts;

@subs = sort {$b->[0] <=> $a->[0]} @subs; # Descending sort by first c
+har pos

for my $sub (@subs) {
    unshift @counts, 1 + (substr $sentence, 0, $sub->[0]) =~ s/(\s+)/$
+1/g;
    substr $sentence, $_, 0, '**' for $sub->[1] + 1, $sub->[0];
}

print "@counts $sentence\n";
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Prints:

4 7 Sam goes to **school** to play **football**.
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sub words_before {
    my ( $word, $string ) = @_;
    my ( $before ) = $string =~ m/(.*?)\Q$word\E/g;
    my $words_before = () = $before =~ m/(\S+)/g;
    return $words_before;
}
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print 'There are ' . words_before( $word, $string ) . ' words before "
+' . $word . '" in the string "' . $string . '"' . "\n";
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Please see How do I post a question effectively? -- specifically, RTFM (Show Some Effort), Do Your Own Work, and About Homework.

I share the suspicions of and disdain for homework not labeled as such expressed by others, but since several code contributions have already been made, here's mine.

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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I'm not sure I understand correctly what you want to do, but I'll try:

my $sentence = 'Sam goes to school to play football.';

substr($sentence, $_, 0) = '**' for 35, 27, 18, 12;

print $sentence, "\n";
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This will print Sam goes to **school** to play **football**., and the asterisks have been put there using the character offsets.

However, a more straightforward way to highlight school this way would be something like:

$sentence =~ s/(school)/**\1**/;
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Update: Sorry, I forgot the part about the word count.. Well, here's a way to get to the "4" (not very nice, maybe others have a more beautiful solution):

my $sentence = 'Sam goes to school to play football.';

# Count the spaces and add 1
my $count1 = substr($sentence, 0, 12) =~ y/ // + 1;
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Dear Monks,

Thanks a lot for your help. This is not a homework problem but part of
+ a bigger project. Here is how I did it:

substr($sentence, $_, 0) = '**' for @temp1; #@temp1 contains the desce
+nding character positions used for marking.
@temp2=split(' ', $sentence); #indices of the elements in @temp2 that 
+have ^\*\* will give me their word count from the start.

New thing that I learned here is that substr() could also be used for 
+assignment...:).

Thank you all beloved monks!
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