my @lines = ( q(String),
              q(I have a String),
              q(I have a String and a .c),
              q(I have a String.c),
              q(String.O) );

foreach (@lines) {
    #Look for String, not followed by .c, .o
    next unless /String(?![.][coCO])/;
    print $_ . "\n";
}

Output:
String
I have a String
I have a String and a .c