Refactor perl sub for testability - perl

Question

I have inherited a perl code base. Consider the following subroutine;
sub getSysRTable
{
my $iface = shift;
return if not length($iface);
my %ip_routes;
my #routes = `/usr/bin/netstat -rn`;
foreach my $route(#routes) {
if ($route =~ /([\S.]+)\s+([\d.]+.[\d.]+.[\d.]+.[\d.]+)\s+(UGS|UGHS)\s+($iface)/ )
{ $ip_routes {$1} = $2 }
}
return %ip_routes;
}
I want to write unit tests for this code. The testing I have in mind will use sample output from netstat -rn and check for expected behaviour. The sub as is, invokes a command, so injecting my test data is problematic with this implementation.
What is the idiomatic perlish approach to refactoring this sub for testability?

Answer 1

First, change your code as follows:
sub getDataForSysRTable {
return `/usr/bin/netstat -rn`;
}
sub getSysRTable
{
my $iface = shift;
return if not length($iface);
my %ip_routes;
my #routes = getDataForSysRTable();
foreach my $route(#routes) {
if ($route =~ /([\S.]+)\s+([\d.]+.[\d.]+.[\d.]+.[\d.]+)\s+(UGS|UGHS)\s+($iface)/ )
{ $ip_routes {$1} = $2 }
}
return %ip_routes;
}
Then for your test, you can do
local *getDataForSysRTable = sub {
... return known data ...
};
my $ip_routes = getSysRTable($iface);