$string = "This is bLarney rubble and his friends joe rockhead and fred flintstone";
$count = 0;

for $target (qw(fred barney joe)) {
    if ( $string =~ /(?=.*\b$target (\w+))/i ) {
        push @elements, $1;
        $count++;
    }
    else {
        push @elements, ''; # as a placeholder
    }
}

if ($count >= 2) { print join('_', @elements), "_inc\n" }
else             { print "Didn't find at least 2 elements in the string\n" }

# prints flintstone__blockhead_inc
# change 'joe' to 'moe' and you get > Didn't find at least 2 elements in the string