use v5.10;
use strict;
use warnings;

package System::Sub::Env;

use Carp qw(croak);
use File::Which qw(which);

use base qw(System::Sub);

sub import {
	my $class = shift;
	my @super;
	while (@_) {
		my $cmd = shift;
		my %opt = @{ ref($_[0]) ? shift : [] };
		my $env = uc("PATH_TO_$cmd");
		$opt{'$0'} //= ($ENV{$env} // which($cmd));
		-x $opt{'$0'}
			or croak("Could not find '$cmd'; please set '$env' environment variable; stopped");
		push @super, $cmd => [%opt];
	}
	@_ = ($class, @super);
	goto \&System::Sub::import;
}

1;

##</code><code>##

use System::Sub::Env "mv";

mv("oldname.txt" => "newname.txt");