You need to escape the + as it is a meta-character which represents the 1 or more quantifier e.g

my $value = 'foo+bar+baz';
$value =~ s/\+/ /g;
print $value;

__output__

foo bar baz
[download]

Also note that I didn't escape the space as it doesn't need to be escaped. See. perlre and perlretut for more info. BTW, if you're trying to parse the key=value pairs from a url then likes of CGI may prove very useful.