Here's one way using a subroutine and the ternary operator. It will return $str if $str isn't empty, and if it is, returns 0.

#!/usr/bin/perl

use warnings;
use strict;

my $string = "data.csv";

my $return = num($string);

sub num {
    my $str = shift;
    $str =~ s/\D//g;

    return $str ? $str : 0;
}
[download]

Or without the sub:

my $str = "data.csv";
$str =~ s/\D//g;
$str = $str ? $str : 0;

print $str;
[download]

-stevieb