Your main question has been answered already but here are a couple of ideas I had that avoid using capturing groups. The first just replaces any '.' with '.0'. The second uses a lookbehind to find a digit and a '.' and a lookahead to check for a digit and then inserts a '0'. I looked around for a numeric version of \b for matching number boundaries but didn't see anything.

use strict;
use warnings;

my $num1 = 2.3;
print "$num1 -> ";
$num1 =~ s/\./.0/;
print "$num1\n";

$num1 = 2.3;
print "$num1 -> ";
$num1 =~ s/(?<=\d\.)(?=\d)/0/;
print "$num1\n";

__DATA__
2.3 -> 2.03
2.3 -> 2.03
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