my $str = "foobar";
my $x = $str=~/[aeiou]/g;
# matches first "o"  => $x is true,  pos($str) is 2
$x = $str=~/[aeiou]/g;
# matches second "o" => $x is true,  pos($str) is 3
$x = $str=~/[aeiou]/g;
# matches "a"        => $x is true,  pos($str) is 5
$x = $str=~/[aeiou]/g;
# no more matches    => $x is false, pos($str) is undef