print $string =~ s/_/ /g;
[download]

That line actually parses to something like:

$matches = $string =~ s/_/ /g;
print $matches;
[download]

That's because print is receiving the result returned by the substitution, not the string that is being substituted. And the result returned in a substitution is the number of successful substtutions.

----
I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
-- Schemer

Note: All code is untested, unless otherwise stated