while (<>) {  # read a string from input
    my @senteces = split /\.\s+/;  # ". " might work for splitting into sentences(?)
    for my $i ( 0 .. $#sentences ) {
        if ( $sentence[$i] =~ /(?<!\S)(f(?:oo|o?u))(?!\S)/ )
            my $match = $1;
            my $position = index( $sentence[$i], $match );
            printf( "%s found in line %d, sentence %d\n",
                    $match, $., $i+1 );
        }
    }
}