There are many variants of how to do this. My inclination might be:

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'
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If the purpose of the loop is to find an index, then I would try to make that fact prominent at the start of the loop. That way you don't have to read the body of the code to figure out the purpose/intent.

In Perl often there are other ways to do something similar to this functionality with the various List::utils. My Perl code rarely uses any $i subscripts.

update: maybe I have an "off by one" error in original post. I think it is also possible to use the C style loop:

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