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";
}