you're already doing that! the backticks (``) capture the output of sed, and place it into $haha. (You're missing a dollar sign at the front of the line.) print($haha) and you'll see the fixed up text :)

$haha = 'I have black tyres.';
$haha=`echo $haha | sed "s/tyre/tire/g"`;
print($haha, "\n");
# outputs: I have black tires.
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However, there's no need to use sed in this fashion. perl is quite good at doing text manipulations, including regular expressions:

$haha = 'I have black tyres.';
$haha =~ s/tyre/tire/g;
print($haha, "\n");
# outputs: I have black tires.
[download]