Usually writing regex's for HTML is a bad idea. In any case, you should be able to modify this to suit your needs.

my $newvalue = "foobar";

foreach (<DATA>) {
    s/(<input .+? value=["']).*?(["'].*?>)/$1$newvalue$2/;
    print;
}

__DATA__
<input type="text" value="">
<input type="password" name="moo" value="fish" />
<input type="text" value=''/>
[download]

Note that it assumes the tag already has a value attribute (either set or empty).