use constant HIGHEST_CHAR_ON_KBD => 126, #These values may differ for you, depending on where you bought
             LOWEST_CHAR_ON_KBD => 9;   #  your keyboard. There are some extra, non-keyboard chars in this range, as well.
while( <FILE> ) {
    foreach( split("", $_) ) {
        if( ( ord($_) > HIGHEST_CHAR_ON_KBD ) || ( ord($_) < LOWEST_CHAR_ON_KBD) ) {
            say "It's a binary file";
            last;
        }
    }
}