It's a conditional ternary operator. You can read about it in perlop. An equivalent to Zaxo's

print exists $search{$_}   ?
          "$_ : Found $_\n"    :
          "$_ : Not Found $_\n";
[download]

would be this:

if (exists $search{$_})
{
    print "$_ : Found $_\n";
}
else
{
    print "$_ : Not Found $_\n";
}
[download]

The result is the same, but the layout is more compact with less duplication.