foo( "one" );
foo( ["two", "three"] );

sub foo {
	my $x = shift;
	# the following does the trick
	my @values = ref $x eq 'ARRAY' ? @{$x} : $x;
	for my $val (@values) {
		print "<$val>\n";
	}
}

__END__

<one>
<two>
<three>