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 char 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";

##</code><code>##

4 7 Sam goes to **school** to play **football**.