calc_string.pl sample code of twice eval simple_replace option. see: https://github.com/perl-entrance-org/workshop-2013-05/blob/master/slide.md

calc_string.pl

#!/usr/bin/env perl
# https://github.com/perl-entrance-org/workshop-2013-05/blob/master/slide.md#%E7%B7%B4%E7%BF%92%E5%95%8F%E9%A1%8C-1

use strict;
use warnings;

while(my $str = <STDIN>) {
    chomp $str;
    my $res = calc_strings($str);
    if ( defined $res ) {
        print "$res\n";
    } else {
        print "Input Error: $str\n";
    }
}

sub calc_strings {
    my $str = shift;
    # 文字クラス [...] の中での - は文字コード範囲になるので端っこに置く
    $str =~ s|^(\d+)\s*([-+/*])\s*(\d+)$| "$1 $2 $3" |ee
        or return undef;
    return $str;
}