#!/usr/bin/perl
use strict;
use warnings;

my $str = 'hello nr34 , 56, hi';
print "input string=$str\n";

$str =~ s/.*nr(\d+)([^\d]+)(\d+)(.*)/"calc(nr$1)$2".eval{$3*4}."$4";/e;
print "output string=$str\n";

__END__
Prints:
input string=hello nr34 , 56, hi
output string=calc(nr34) , 224, hi