Here's another option using Regexp::Common:

use strict;
use warnings;
use Regexp::Common qw/ number /;

print for grep /^$RE{num}{int}$/, <DATA>;

__DATA__
3.14
10
-42
42
.14159
12.34e-56
3.0
Hello, world.
[download]

Output:

10
-42
42
[download]