I think what you're confused with is matching against the special variable $_, not list context vs scalar context. Note that this:

$_ = "1.0 and 2.4 and 310 and 4.7 and so on";
@a = m/([\d|\.]+)\D+/g;
print "@a\n";

#result
#1.0 2.4 310 4.7
[download]

can be rewritten as this

$_ = "1.0 and 2.4 and 310 and 4.7 and so on";
@a = ($_ =~ m/([\d|\.]+)\D+/g);
print "@a\n";

#result
#1.0 2.4 310 4.7
[download]

without changing the meaning whatsoever. Your two examples only differ in that the first matches against a defined scalar, whereas the second matches against $_.