say "$_ => [", (m/(\d*\.?+\d*)/x ? "$1]" : ']');
#                     ^

##</code><code>##

#! perl
use strict;
use warnings;
use feature qw( say );

my $decimal = qr{
                    (
                        (?: \d+ \.? \d* ) |
                        (?: \d* \.? \d+ )
                    )
                }x;

while (<DATA>)
{
    chomp;
    next unless length;
    say "$_ => [", (/$decimal/x ? "$1]" : ']');
}

__DATA__
0.12
.12
12.
12
no numbers here
abc.def42
.7zx

##</code><code>##

16:39 >perl 1621_SoPW.pl
0.12 => [0.12]
.12 => [.12]
12. => [12.]
12 => [12]
no numbers here => []
abc.def42 => [42]
.7zx => [.7]

16:39 >