my $interesting_index;
my $i = 0;

while (!defined($interesting_index) and $i++ <= 10)
{   
    $interesting_index = $i if condition($i);
}

# spit an error if $interesting_index is still 'not defined'

##</code><code>##

for (my $i=1; !defined($interesting_index) and $i<=10; $i++){}