On the most recent perl's you can use each:

my @array = (2,4,7,5,8);;
my($i, $v ) = each @array until $v == 7;;
print $i;;
2
[download]

On older perl's:

my @array = (2,4,7,5,8);;
my $i = 0;;
++$i until $array[ $i ] == 7;;
print $i;;
2
[download]