Assuming (dangerous) that I've interpreted your problem statement as you intended, one way of doing the job (in the problem you specified; adding 10 to a 3 digit decimal value) and without resort to executing code inside a regex is:
#/usr/bin/perl
use strict;
use warnings;
# 892837
my $x = 123; # no quotes
my $y = $x;
$y =~ /(\d{2})(\d)/; # regex, but see also perlretut re executing c
+ode in a regex
$y = ($1+1).$2;
print $y;