Actually, I'm not sure that iterating through the array is such a bad option, particularly if you're going to exit the loop after you find an element. If you use grep, you're going to look through all of the items in the array no matter what; what if the first element in the array matches, and you have 100 items? Then you're looking through 99 items when you don't need to. That's an exaggeration, but the point is the same.

The hash option is a good one, although not if you're actually looking through each item for regular expression matches--are you? It seems like you could be from your explanation, but I'm not absolutely sure that you are. In that case, you probably can't use a hash...

meaning that, in my opinion, your best option is to iterate through the array and check each element, then exit the loop once you've found a matching element.

my $itemPart = "Group1]Item2";
my @items = (
    '[x.Group1]Item1=Value1',
    '[y.Group1]Item2=Value2',
    '[z.Group1]Item3=Value3',
    '[a.Group2]Item1=Value1',
    '[b.Group2]Item2=Value2'
);

my $found_item;
for my $item (@items) {
    # for each element, test if it contains $itemPart;
    # we need to use \Q and \E to meta-quote $itemPart,
    # in case it contains any regexp metacharacters
    if ($item =~ /\Q$itemPart\E/o) {
        $found_item = $item;
        last;
    }
}

# now check if we actually found an item;
# if we did, $found_item will be defined
if (defined $found_item) {
    print $found_item, "\n";
}
else {
    print "Not Found!\n";
}
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It's possible, of course, that you want *all* items that match in the array; in that case, use grep:

my @matching = grep /\Q$itemPart\E/o, @items;
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